(**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *)

module Ast = Flow_ast

let ( * ) : 'a 'b 'c 'd . ('a -> 'c) -> ('b -> 'd) -> ('a * 'b) -> ('c * 'd) =
  fun f g (x, y) -> (f x, g y)

let id : 'a . 'a -> 'a = fun x -> x

class virtual ['M, 'T, 'N, 'U] mapper = object(this)

  method virtual on_loc_annot : 'M -> 'N
  method virtual on_type_annot : 'T -> 'U

  method program (program: ('M, 'T) Ast.program) : ('N, 'U) Ast.program =
    let (annot, statements, comments) = program in
    let annot' = this#on_loc_annot annot in
    let statements' = this#toplevel_statement_list statements in
    let comments' = Core_list.map ~f:(this#comment) comments in
    annot', statements', comments'

  method statement ((annot, stmt): ('M, 'T) Ast.Statement.t) : ('N, 'U) Ast.Statement.t =
    let open Ast.Statement in
    this#on_loc_annot annot,
    match stmt with
    | Block block -> Block (this#block block)

    | Break break -> Break (this#break break)

    | ClassDeclaration cls -> ClassDeclaration (this#class_ cls)

    | Continue cont -> Continue (this#continue cont)

    | Debugger -> Debugger

    | DeclareClass stuff -> DeclareClass (this#declare_class stuff)

    | DeclareExportDeclaration decl ->
      DeclareExportDeclaration (this#declare_export_declaration annot decl)

    | DeclareFunction stuff -> DeclareFunction (this#declare_function stuff)

    | DeclareInterface stuff -> DeclareInterface (this#declare_interface stuff)

    | DeclareModule m -> DeclareModule (this#declare_module annot m)

    | DeclareTypeAlias stuff -> DeclareTypeAlias (this#declare_type_alias stuff)

    | DeclareVariable stuff -> DeclareVariable (this#declare_variable stuff)

    | DeclareModuleExports t_annot ->
      DeclareModuleExports (this#declare_module_exports annot t_annot)

    | DoWhile stuff -> DoWhile (this#do_while stuff)

    | Empty ->
      this#empty ();
      Empty

    | ExportDefaultDeclaration decl ->
      ExportDefaultDeclaration (this#export_default_declaration annot decl)

    | ExportNamedDeclaration decl ->
      ExportNamedDeclaration (this#export_named_declaration annot decl)

    | Expression expr -> Expression (this#expression_statement expr)

    | For for_stmt -> For (this#for_statement for_stmt)

    | ForIn stuff -> ForIn (this#for_in_statement stuff)

    | ForOf stuff -> ForOf (this#for_of_statement stuff)

    | FunctionDeclaration func ->
      FunctionDeclaration (this#function_declaration func)

    | If if_stmt -> If (this#if_statement if_stmt)

    | ImportDeclaration decl ->
      ImportDeclaration (this#import_declaration annot decl)

    | InterfaceDeclaration stuff ->
      InterfaceDeclaration (this#interface_declaration stuff)

    | Labeled label -> Labeled (this#labeled_statement label)

    | OpaqueType otype -> OpaqueType (this#opaque_type otype)

    | Return ret -> Return (this#return ret)

    | Switch switch -> Switch (this#switch switch)

    | Throw throw -> Throw (this#throw throw)

    | Try try_stmt -> Try (this#try_catch try_stmt)

    | VariableDeclaration decl ->
      VariableDeclaration (this#variable_declaration decl)

    | While stuff -> While (this#while_ stuff)

    | With stuff -> With (this#with_ stuff)

    | TypeAlias stuff -> TypeAlias (this#type_alias stuff)

    | DeclareOpaqueType otype ->
      DeclareOpaqueType (this#declare_opaque_type otype)

  method comment ((annot, c): 'M Ast.Comment.t) : 'N Ast.Comment.t =
    this#on_loc_annot annot, c

  method expression ((annot, expr'): ('M, 'T) Ast.Expression.t) : ('N, 'U) Ast.Expression.t =
    let open Ast.Expression in
    this#on_type_annot annot,
    match expr' with
    | This -> This
    | Super -> Super
    | Array x -> Array (this#array x)
    | ArrowFunction x -> ArrowFunction (this#arrow_function x)
    | Assignment x -> Assignment (this#assignment x)
    | Binary x -> Binary (this#binary x)
    | Call x -> Call (this#call annot x)
    | Class x -> Class (this#class_ x)
    | Comprehension x -> Comprehension (this#comprehension x)
    | Conditional x -> Conditional (this#conditional x)
    | Function x -> Function (this#function_ x)
    | Generator x -> Generator (this#generator x)
    | Identifier x -> Identifier (this#t_identifier x)
    | Import x -> Import (this#import annot x)
    | JSXElement x -> JSXElement (this#jsx_element x)
    | JSXFragment x -> JSXFragment (this#jsx_fragment x)
    | Literal x -> Literal (this#literal x)
    | Logical x -> Logical (this#logical x)
    | Member x -> Member (this#member x)
    | MetaProperty x -> MetaProperty (this#meta_property x)
    | New x -> New (this#new_ x)
    | Object x -> Object (this#object_ x)
    | OptionalCall x -> OptionalCall (this#optional_call annot x)
    | OptionalMember x -> OptionalMember (this#optional_member x)
    | Sequence x -> Sequence (this#sequence x)
    | TaggedTemplate x -> TaggedTemplate (this#tagged_template x)
    | TemplateLiteral x -> TemplateLiteral (this#template_literal x)
    | TypeCast x -> TypeCast (this#type_cast x)
    | Unary x -> Unary (this#unary_expression x)
    | Update x -> Update (this#update_expression x)
    | Yield x -> Yield (this#yield x)

  method array (expr: ('M, 'T) Ast.Expression.Array.t) : ('N, 'U) Ast.Expression.Array.t =
    let open Ast.Expression in
    let { Array.elements } = expr in
    let elements' = Core_list.map ~f:(Option.map ~f:this#expression_or_spread) elements in
    { Array.elements = elements' }

  method arrow_function (expr: ('M, 'T) Ast.Function.t) : ('N, 'U) Ast.Function.t =
    this#function_ expr

  method assignment (expr: ('M, 'T) Ast.Expression.Assignment.t)
                        : ('N, 'U) Ast.Expression.Assignment.t =
    let open Ast.Expression.Assignment in
    let { operator; left; right } = expr in
    let left' = this#assignment_pattern left in
    let right' = this#expression right in
    { operator; left = left'; right = right' }

  method binary (expr: ('M, 'T) Ast.Expression.Binary.t) : ('N, 'U) Ast.Expression.Binary.t =
    let open Ast.Expression.Binary in
    let { operator; left; right } = expr in
    let left' = this#expression left in
    let right' = this#expression right in
    { operator; left = left'; right = right' }

  method block (stmt: ('M, 'T) Ast.Statement.Block.t) : ('N, 'U) Ast.Statement.Block.t =
    let open Ast.Statement.Block in
    let { body } = stmt in
    let body' = this#statement_list body in
    { body = body' }

  method break (break: 'M Ast.Statement.Break.t) : 'N Ast.Statement.Break.t =
    let open Ast.Statement.Break in
    let { label } = break in
    let label' = Option.map ~f:this#label_identifier label in
    { label = label' }

  method call _annot (expr: ('M, 'T) Ast.Expression.Call.t) : ('N, 'U) Ast.Expression.Call.t =
    let open Ast.Expression.Call in
    let { callee; targs; arguments } = expr in
    let callee' = this#expression callee in
    let targs' = Option.map ~f:this#type_parameter_instantiation_with_implicit targs in
    let arguments' = Core_list.map ~f:this#expression_or_spread arguments in
    { callee = callee'; targs = targs'; arguments = arguments' }

  method optional_call annot (expr: ('M, 'T) Ast.Expression.OptionalCall.t)
                                : ('N, 'U) Ast.Expression.OptionalCall.t =
    let open Ast.Expression.OptionalCall in
    let { call; optional; } = expr in
    let call' = this#call annot call in
    { call = call'; optional }

  method catch_clause (clause: ('M, 'T) Ast.Statement.Try.CatchClause.t')
                            : ('N, 'U) Ast.Statement.Try.CatchClause.t' =
    let open Ast.Statement.Try.CatchClause in
    let { param; body } = clause in
    let param' = Option.map ~f:this#catch_clause_pattern param in
    let body' = (this#on_loc_annot * this#block) body in
    { param = param'; body = body' }

  method class_ (cls: ('M, 'T) Ast.Class.t) : ('N, 'U) Ast.Class.t =
    let open Ast.Class in
    let { id; body; tparams; extends; implements; classDecorators; } = cls in
    let id' = Option.map ~f:this#class_identifier id in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let extends' = Option.map ~f:this#class_extends extends in
      let body' = this#class_body body in
      let implements' = Core_list.map ~f:this#class_implements implements in
      let classDecorators' = Core_list.map ~f:this#class_decorator classDecorators in
      {
        id = id';
        body = body';
        tparams = tparams';
        extends = extends';
        implements = implements';
        classDecorators = classDecorators';
      }
    )

  method class_extends (extends : ('M, 'T) Ast.Class.Extends.t) : ('N, 'U) Ast.Class.Extends.t =
    let open Ast.Class.Extends in
    let annot, { expr; targs } = extends in
    let expr' = this#expression expr in
    let targs' = Option.map ~f:this#type_parameter_instantiation targs in
    this#on_loc_annot annot, { expr = expr'; targs = targs' }

  method class_decorator (dec : ('M, 'T) Ast.Class.Decorator.t) : ('N, 'U) Ast.Class.Decorator.t =
    let open Ast.Class.Decorator in
    let annot, { expression } = dec in
    let expression' = this#expression expression in
    this#on_loc_annot annot, { expression = expression' }

  method class_identifier (ident: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#t_pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Let ident

  method class_body (cls_body: ('M, 'T) Ast.Class.Body.t) : ('N, 'U) Ast.Class.Body.t =
    let open Ast.Class.Body in
    let annot, { body } = cls_body in
    let body' = Core_list.map ~f:this#class_element body in
    this#on_type_annot annot, { body = body' }

  method class_element (elem: ('M, 'T) Ast.Class.Body.element) : ('N, 'U) Ast.Class.Body.element =
    let open Ast.Class.Body in
    match elem with
    | Method (annot, meth) ->
      Method (this#on_type_annot annot, this#class_method meth)
    | Property (annot, prop) ->
      Property (this#on_type_annot annot, this#class_property prop)
    | PrivateField (annot, field) ->
      PrivateField (this#on_type_annot annot, this#class_private_field field)

  method class_method (meth: ('M, 'T) Ast.Class.Method.t') : ('N, 'U) Ast.Class.Method.t' =
    let open Ast.Class.Method in
    let { kind; key; value; static; decorators; } = meth in
    let key' = this#object_key key in
    let value' = (this#on_loc_annot * this#function_) value in
    let decorators' = Core_list.map ~f:this#class_decorator decorators in
    { kind; key = key'; value = value'; static; decorators = decorators' }

  method class_property (prop: ('M, 'T) Ast.Class.Property.t') : ('N, 'U) Ast.Class.Property.t' =
    let open Ast.Class.Property in
    let { key; value; annot; static; variance; } = prop in
    let key' = this#object_key key in
    let value' = Option.map ~f:this#expression value in
    let annot' = this#type_annotation_hint annot in
    let variance' = Option.map ~f:(this#on_loc_annot * id) variance in
    { key = key'; value = value'; annot = annot'; static; variance = variance'; }

  method class_private_field (prop: ('M, 'T) Ast.Class.PrivateField.t') =
    let open Ast.Class.PrivateField in
    let { key; value; annot; static; variance; } = prop in
    let key' = this#private_name key in
    let value' = Option.map ~f:this#expression value in
    let annot' = this#type_annotation_hint annot in
    let variance' = Option.map ~f:(this#on_loc_annot * id) variance in
    { key = key'; value = value'; annot = annot'; static; variance = variance' }

  method comprehension (expr: ('M, 'T) Ast.Expression.Comprehension.t)
                            : ('N, 'U) Ast.Expression.Comprehension.t =
    let open Ast.Expression.Comprehension in
    let { blocks; filter } = expr in
    let blocks' = Core_list.map ~f:this#comprehension_block blocks in
    let filter' = Option.map ~f:this#expression filter in
    { blocks = blocks'; filter = filter' }

  method comprehension_block (block : ('M, 'T) Ast.Expression.Comprehension.Block.t)
                                    : ('N, 'U) Ast.Expression.Comprehension.Block.t =
    let open Ast.Expression.Comprehension.Block in
    let annot, { left; right; each } = block in
    let left' = this#pattern left in
    let right' = this#expression right in
    this#on_loc_annot annot, { left = left'; right = right'; each }

  method conditional (expr: ('M, 'T) Ast.Expression.Conditional.t)
                          : ('N, 'U) Ast.Expression.Conditional.t =
    let open Ast.Expression.Conditional in
    let { test; consequent; alternate } = expr in
    let test' = this#predicate_expression test in
    let consequent' = this#expression consequent in
    let alternate' = this#expression alternate in
    { test = test'; consequent = consequent'; alternate = alternate' }

  method continue (cont: 'M Ast.Statement.Continue.t) : 'N Ast.Statement.Continue.t =
    let open Ast.Statement.Continue in
    let { label } = cont in
    let label' = Option.map ~f:this#label_identifier label in
    { label = label' }

  method debugger () =
    ()

  method declare_class (decl: ('M, 'T) Ast.Statement.DeclareClass.t)
                            : ('N, 'U) Ast.Statement.DeclareClass.t =
    let open Ast.Statement.DeclareClass in
    let { id = ident; tparams; body; extends; mixins; implements } = decl in
    let id' = this#class_identifier ident in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let body' =
        let a, b = body in
        this#on_loc_annot a, this#object_type b in
      let extends' = Option.map ~f:(this#on_loc_annot * this#generic_type) extends in
      let mixins' = Core_list.map ~f:(this#on_loc_annot * this#generic_type) mixins in
      let implements' = Core_list.map ~f:this#class_implements implements in
      {
        id = id';
        tparams = tparams';
        body = body';
        extends = extends';
        mixins = mixins';
        implements = implements';
      }
    )

  method class_implements (implements: ('M, 'T) Ast.Class.Implements.t)
                                    : ('N, 'U) Ast.Class.Implements.t =
    let open Ast.Class.Implements in
    let annot, { id = id_; targs } = implements in
    let id' = this#t_identifier id_ in
    let targs' = Option.map ~f:this#type_parameter_instantiation targs in
    this#on_loc_annot annot, { id = id'; targs = targs' }

  method declare_export_declaration _annot
    (decl: ('M, 'T) Ast.Statement.DeclareExportDeclaration.t)
         : ('N, 'U) Ast.Statement.DeclareExportDeclaration.t =
    let open Ast.Statement.DeclareExportDeclaration in
    let { default; source; specifiers; declaration } = decl in
    let default' = Option.map ~f:this#on_loc_annot default in
    let source' = Option.map ~f:(this#on_loc_annot * id) source in
    let specifiers' = Option.map ~f:this#export_named_specifier specifiers in
    let declaration' = Option.map ~f:this#declare_export_declaration_decl declaration in
    {
      default = default';
      source = source';
      specifiers = specifiers';
      declaration = declaration'
    }

  method declare_export_declaration_decl
    (decl: ('M, 'T) Ast.Statement.DeclareExportDeclaration.declaration)
        : ('N, 'U) Ast.Statement.DeclareExportDeclaration.declaration =
    let open Ast.Statement.DeclareExportDeclaration in
    match decl with
    | Variable (annot, decl_var) ->
      Variable (this#on_loc_annot annot, this#declare_variable decl_var)
    | Function (annot, decl_func) ->
      Function (this#on_loc_annot annot, this#declare_function decl_func)
    | Class (annot, decl_class) ->
      Class (this#on_loc_annot annot, this#declare_class decl_class)
    | DefaultType t ->
      DefaultType (this#type_ t)
    | NamedType (annot, alias) ->
      NamedType (this#on_loc_annot annot, this#type_alias alias)
    | NamedOpaqueType (annot, ot) ->
      NamedOpaqueType (this#on_loc_annot annot, this#opaque_type ot)
    | Interface (annot, iface) ->
      Interface (this#on_loc_annot annot, this#interface iface)

  method declare_function (decl: ('M, 'T) Ast.Statement.DeclareFunction.t)
                              : ('N, 'U) Ast.Statement.DeclareFunction.t =
    let open Ast.Statement.DeclareFunction in
    let { id = ident; annot; predicate } = decl in
    let id' = this#function_identifier ident in
    let annot' = this#type_annotation annot in
    let predicate' = Option.map ~f:this#type_predicate predicate in
    { id = id'; annot = annot'; predicate = predicate' }

  method declare_interface (decl: ('M, 'T) Ast.Statement.Interface.t)
                                : ('N, 'U) Ast.Statement.Interface.t =
    this#interface decl

  method declare_module _annot (m: ('M, 'T) Ast.Statement.DeclareModule.t)
                              : ('N, 'U) Ast.Statement.DeclareModule.t =
    let open Ast.Statement.DeclareModule in
    let { id; body; kind } = m in
    let id' = match id with
      | Identifier (annot, name) -> Identifier (this#on_type_annot annot, name)
      | Literal (annot, name) -> Literal (this#on_type_annot annot, name)
    in
    let kind' = match kind with
      | CommonJS annot -> CommonJS (this#on_loc_annot annot)
      | ES annot -> ES (this#on_loc_annot annot)
    in
    let body' = (this#on_loc_annot * this#block) body in
    { id = id'; body = body'; kind = kind' }

  method declare_module_exports _annot (t_annot: ('M, 'T) Ast.Type.annotation)
                                              : ('N, 'U) Ast.Type.annotation =
    this#type_annotation t_annot

  method declare_type_alias (decl: ('M, 'T) Ast.Statement.TypeAlias.t)
                                : ('N, 'U) Ast.Statement.TypeAlias.t =
    this#type_alias decl

  method declare_variable (decl: ('M, 'T) Ast.Statement.DeclareVariable.t)
                              : ('N, 'U) Ast.Statement.DeclareVariable.t =
    let open Ast.Statement.DeclareVariable in
    let { id = ident; annot } = decl in
    let id' = this#t_pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Var ident in
    let annot' = this#type_annotation_hint annot in
    { id = id'; annot = annot' }

  method do_while (stuff: ('M, 'T) Ast.Statement.DoWhile.t) : ('N, 'U) Ast.Statement.DoWhile.t =
    let open Ast.Statement.DoWhile in
    let { body; test } = stuff in
    let body' = this#statement body in
    let test' = this#predicate_expression test in
    { body = body'; test = test' }

  method empty () =
    ()

  method export_default_declaration _loc (decl: ('M, 'T) Ast.Statement.ExportDefaultDeclaration.t)
                                              : ('N, 'U) Ast.Statement.ExportDefaultDeclaration.t =
    let open Ast.Statement.ExportDefaultDeclaration in
    let { default; declaration } = decl in
    let default' = this#on_loc_annot default in
    let declaration' = this#export_default_declaration_decl declaration in
    { default = default'; declaration = declaration' }

  method export_default_declaration_decl
      (decl: ('M, 'T) Ast.Statement.ExportDefaultDeclaration.declaration)
          : ('N, 'U) Ast.Statement.ExportDefaultDeclaration.declaration =
    let open Ast.Statement.ExportDefaultDeclaration in
    match decl with
    | Declaration stmt -> Declaration (this#statement stmt)
    | Expression expr -> Expression (this#expression expr)

  method export_named_declaration _loc (decl: ('M, 'T) Ast.Statement.ExportNamedDeclaration.t)
                                            : ('N, 'U) Ast.Statement.ExportNamedDeclaration.t =
    let open Ast.Statement.ExportNamedDeclaration in
    let { exportKind; source; specifiers; declaration } = decl in
    let source' = Option.map ~f:(this#on_loc_annot * id) source in
    let specifiers' = Option.map ~f:this#export_named_specifier specifiers in
    let declaration' = Option.map ~f:this#statement declaration in
    { exportKind; source = source'; specifiers = specifiers'; declaration = declaration' }

  method export_named_specifier (spec: 'M Ast.Statement.ExportNamedDeclaration.specifier)
                                    : 'N Ast.Statement.ExportNamedDeclaration.specifier =
    let open Ast.Statement.ExportNamedDeclaration in
    match spec with
    | ExportSpecifiers specs -> ExportSpecifiers (Core_list.map ~f:this#export_specifier specs)
    | ExportBatchSpecifier (annot, name) ->
      let annot' = this#on_loc_annot annot in
      let name' = Option.map ~f:(this#on_loc_annot * id) name in
      ExportBatchSpecifier (annot', name')

  method export_specifier (spec : 'M Ast.Statement.ExportNamedDeclaration.ExportSpecifier.t)
                                : 'N Ast.Statement.ExportNamedDeclaration.ExportSpecifier.t =
    let open Ast.Statement.ExportNamedDeclaration.ExportSpecifier in
    let annot, { local; exported } = spec in
    let local' = (this#on_loc_annot * id) local in
    let exported' = Option.map ~f:(this#on_loc_annot * id) exported in
    this#on_loc_annot annot, { local = local'; exported = exported' }

  method expression_statement (stmt: ('M, 'T) Ast.Statement.Expression.t)
                                  : ('N, 'U) Ast.Statement.Expression.t =
    let open Ast.Statement.Expression in
    let { expression = expr; directive; } = stmt in
    { expression = this#expression expr; directive; }

  method expression_or_spread expr_or_spread =
    let open Ast.Expression in
    match expr_or_spread with
    | Expression expr ->
      Expression (this#expression expr)
    | Spread spread ->
      Spread (this#spread_element spread)

  method for_in_statement (stmt: ('M, 'T) Ast.Statement.ForIn.t) : ('N, 'U) Ast.Statement.ForIn.t =
    let open Ast.Statement.ForIn in
    let { left; right; body; each } = stmt in
    let left' = this#for_in_statement_lhs left in
    let right' = this#expression right in
    let body' = this#statement body in
    { left = left'; right = right'; body = body'; each }

  method for_in_statement_lhs (left: ('M, 'T) Ast.Statement.ForIn.left)
                                  : ('N, 'U) Ast.Statement.ForIn.left =
    let open Ast.Statement.ForIn in
    match left with
    | LeftDeclaration (annot, decl) ->
      LeftDeclaration (this#on_loc_annot annot, this#variable_declaration decl)
    | LeftPattern patt -> LeftPattern (this#for_in_assignment_pattern patt)

  method for_of_statement (stuff: ('M, 'T) Ast.Statement.ForOf.t) : ('N, 'U) Ast.Statement.ForOf.t =
    let open Ast.Statement.ForOf in
    let { left; right; body; async } = stuff in
    let left' = this#for_of_statement_lhs left in
    let right' = this#expression right in
    let body' = this#statement body in
    { left = left'; right = right'; body = body'; async }

  method for_of_statement_lhs (left: ('M, 'T) Ast.Statement.ForOf.left) =
    let open Ast.Statement.ForOf in
    match left with
    | LeftDeclaration (annot, decl) ->
      LeftDeclaration (this#on_loc_annot annot, this#variable_declaration decl)
    | LeftPattern patt ->
      LeftPattern (this#for_of_assignment_pattern patt)

  method for_statement (stmt: ('M, 'T) Ast.Statement.For.t) : ('N, 'U) Ast.Statement.For.t =
    let open Ast.Statement.For in
    let { init; test; update; body } = stmt in
    let init' = Option.map ~f:this#for_statement_init init in
    let test' = Option.map ~f:this#predicate_expression test in
    let update' = Option.map ~f:this#expression update in
    let body' = this#statement body in
    { init = init'; test = test'; update = update'; body = body' }

  method for_statement_init (init: ('M, 'T) Ast.Statement.For.init)
                                : ('N, 'U) Ast.Statement.For.init =
    let open Ast.Statement.For in
    match init with
    | InitDeclaration (annot, decl) ->
      InitDeclaration (this#on_loc_annot annot, this#variable_declaration decl)
    | InitExpression expr ->
      InitExpression (this#expression expr)

  method function_param_type (fpt: ('M, 'T) Ast.Type.Function.Param.t)
                                : ('N, 'U) Ast.Type.Function.Param.t =
    let open Ast.Type.Function.Param in
    let annot, { annot = t_annot; name; optional; } = fpt in
    let t_annot' = this#type_ t_annot in
    let name' = Option.map ~f:this#t_identifier name in
    this#on_loc_annot annot, { annot = t_annot'; name = name'; optional }

  method function_rest_param_type (frpt: ('M, 'T) Ast.Type.Function.RestParam.t)
                                      : ('N, 'U) Ast.Type.Function.RestParam.t =
    let open Ast.Type.Function.RestParam in
    let annot, { argument } = frpt in
    let argument' = this#function_param_type argument in
    this#on_loc_annot annot, { argument = argument' }

  method function_type (ft: ('M, 'T) Ast.Type.Function.t) : ('N, 'U) Ast.Type.Function.t =
    let open Ast.Type.Function in
    let {
      params = (params_annot, { Params.params = ps; rest = rpo });
      return;
      tparams;
    } = ft in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let ps' = Core_list.map ~f:this#function_param_type ps in
      let rpo' = Option.map ~f:this#function_rest_param_type rpo in
      let return' = this#type_ return in
      {
        params = (this#on_loc_annot params_annot, { Params.params = ps'; rest = rpo' });
        return = return';
        tparams = tparams';
      }
    )

  method label_identifier (ident: 'M Ast.Identifier.t) : 'N Ast.Identifier.t =
    this#identifier ident

  method object_property_value_type (opvt: ('M, 'T) Ast.Type.Object.Property.value)
                                        : ('N, 'U) Ast.Type.Object.Property.value =
    let open Ast.Type.Object.Property in
    match opvt with
    | Init t -> Init (this#type_ t)
    | Get (annot, ft) ->
      Get (this#on_loc_annot annot, this#function_type ft)
    | Set (annot, ft) ->
      Set (this#on_loc_annot annot, this#function_type ft)

  method object_property_type (opt: ('M, 'T) Ast.Type.Object.Property.t)
                                  : ('N, 'U) Ast.Type.Object.Property.t =
    let open Ast.Type.Object.Property in
    let annot, { key; value; optional; static; proto; _method; variance; } = opt in
    let key' = this#object_key key in
    let value' = this#object_property_value_type value in
    let variance' = Option.map ~f:(this#on_loc_annot * id) variance in
    this#on_loc_annot annot, { key = key'; value = value'; optional; static; proto; _method; variance = variance' }

  method object_type (ot: ('M, 'T) Ast.Type.Object.t) : ('N, 'U) Ast.Type.Object.t =
    let open Ast.Type.Object in
    let { properties ; exact; inexact } = ot in
    let properties' = Core_list.map ~f:this#object_type_property properties in
    { properties = properties'; exact; inexact }

  method object_type_property (prop : ('M, 'T) Ast.Type.Object.property)
                                    : ('N, 'U) Ast.Type.Object.property =
    let open Ast.Type.Object in
    match prop with
    | Property prop -> Property (this#object_property_type prop)
    | SpreadProperty (annot, { SpreadProperty.argument }) ->
      let argument' = this#type_ argument in
      SpreadProperty (this#on_loc_annot annot, { SpreadProperty.argument = argument' })
    | Indexer (annot, indexer) ->
      let open Indexer in
      let { id = id_; key; value; static; variance } = indexer in
      let id' = Option.map ~f:(this#on_loc_annot * id) id_ in
      let key' = this#type_ key in
      let value' = this#type_ value in
      let variance' = Option.map ~f:(this#on_loc_annot * id) variance in
      Indexer (this#on_loc_annot annot, { id = id'; key = key'; value = value'; static; variance = variance' })
    | CallProperty (annot, { CallProperty.value; static }) ->
      let open CallProperty in
      let value' = (this#on_loc_annot * this#function_type) value in
      CallProperty (this#on_loc_annot annot, { value = value'; static })
    | InternalSlot (annot, islot) ->
      let open InternalSlot in
      let { id = id_; value; _ } = islot in
      let id' = (this#on_loc_annot * id) id_ in
      let value' = this#type_ value in
      InternalSlot (this#on_loc_annot annot, { islot with id = id'; value = value' })

  method interface_type (i: ('M, 'T) Ast.Type.Interface.t) : ('N, 'U) Ast.Type.Interface.t =
    let open Ast.Type.Interface in
    let { extends; body } = i in
    let extends' = Core_list.map ~f:(this#on_loc_annot * this#generic_type) extends in
    let body' = (this#on_loc_annot * this#object_type) body in
    { extends = extends'; body = body' }

  method generic_identifier_type (git: ('M, 'T)  Ast.Type.Generic.Identifier.t)
                                    : ('N, 'U) Ast.Type.Generic.Identifier.t =
    let open Ast.Type.Generic.Identifier in
    match git with
    | Unqualified i -> Unqualified (this#t_identifier i)
    | Qualified (annot, { qualification; id = id_ }) ->
      let qualification' = this#generic_identifier_type qualification in
      let id' = this#t_identifier id_ in
      Qualified (this#on_loc_annot annot, { qualification = qualification'; id = id' })

  method type_parameter_instantiation (pi: ('M, 'T) Ast.Type.ParameterInstantiation.t)
                                       : ('N, 'U) Ast.Type.ParameterInstantiation.t =
    let annot, targs = pi in
    let targs' = Core_list.map ~f:this#type_ targs in
    this#on_loc_annot annot, targs'

  method type_parameter_instantiation_with_implicit
    (pi: ('M, 'T) Ast.Expression.TypeParameterInstantiation.t)
    : ('N, 'U) Ast.Expression.TypeParameterInstantiation.t =
    let annot, targs = pi in
    let targs' = Core_list.map ~f:this#explicit_or_implicit targs in
    this#on_loc_annot annot, targs'

  method type_parameter_declaration_opt :
    'a . ('M, 'T) Ast.Type.ParameterDeclaration.t option ->
        (('N, 'U) Ast.Type.ParameterDeclaration.t option -> 'a) -> 'a =
      fun pd f ->
        let pd' =
          Option.map ~f:(fun pd ->
            let annot, type_params = pd in
            let type_params' = Core_list.map ~f:this#type_parameter_declaration_type_param type_params in
            this#on_loc_annot annot, type_params'
          ) pd
        in
        f pd'

  method type_parameter_declaration_type_param
    (type_param: ('M, 'T) Ast.Type.ParameterDeclaration.TypeParam.t)
              : ('N, 'U) Ast.Type.ParameterDeclaration.TypeParam.t =
    let open Ast.Type.ParameterDeclaration.TypeParam in
    let annot, { name; bound; variance; default; } = type_param in
    let name' = this#t_identifier name in
    let bound' = this#type_annotation_hint bound in
    let variance' = Option.map ~f:(this#on_loc_annot * id) variance in
    let default' = Option.map ~f:this#type_ default in
    this#on_type_annot annot, { name = name'; bound = bound'; variance = variance'; default = default'; }

  method generic_type (gt: ('M, 'T) Ast.Type.Generic.t) : ('N, 'U) Ast.Type.Generic.t =
    let open Ast.Type.Generic in
    let { id; targs; } = gt in
    let id' = this#generic_identifier_type id in
    let targs' = Option.map ~f:this#type_parameter_instantiation targs in
    { id = id'; targs = targs' }

  method type_predicate ((annot, pred) : ('M, 'T) Ast.Type.Predicate.t)
                                      : ('N, 'U) Ast.Type.Predicate.t =
    let open Ast.Type.Predicate in
    this#on_loc_annot annot,
    match pred with
    | Declared e -> Declared (this#expression e)
    | Inferred -> Inferred

  method type_ ((annot, t): ('M, 'T) Ast.Type.t) : ('N, 'U) Ast.Type.t =
    let open Ast.Type in
    this#on_type_annot annot,
    match t with
    ( Any
    | Mixed
    | Empty
    | Void
    | Null
    | Number
    | String
    | Boolean
    | StringLiteral _
    | NumberLiteral _
    | BooleanLiteral _
    | Exists
    ) as t -> t
    | Nullable t' -> Nullable (this#type_ t')
    | Array t' -> Array (this#type_ t')
    | Typeof t' -> Typeof (this#type_ t')
    | Function ft -> Function (this#function_type ft)
    | Object ot -> Object (this#object_type ot)
    | Interface i -> Interface (this#interface_type i)
    | Generic gt -> Generic (this#generic_type gt)
    | Union (t0, t1, ts) ->
      let t0' = this#type_ t0 in
      let t1' = this#type_ t1 in
      let ts' = Core_list.map ~f:this#type_ ts in
      Union (t0', t1', ts')
    | Intersection (t0, t1, ts) ->
      let t0' = this#type_ t0 in
      let t1' = this#type_ t1 in
      let ts' = Core_list.map ~f:this#type_ ts in
      Intersection (t0', t1', ts')
    | Tuple ts ->
      let ts' = Core_list.map ~f:this#type_ ts in
      Tuple ts'

  method implicit (t: 'T): 'U = this#on_type_annot t
  method explicit_or_implicit
    (x: ('M, 'T) Ast.Expression.TypeParameterInstantiation.type_parameter_instantiation)
    : ('N, 'U) Ast.Expression.TypeParameterInstantiation.type_parameter_instantiation
    =
      let open Ast.Expression.TypeParameterInstantiation in
      match x with
      | Explicit t -> Explicit (this#type_ t)
      | Implicit t -> Implicit (this#implicit t)

  method type_annotation ((annot, t_annot): ('M, 'T) Ast.Type.annotation) =
    this#on_loc_annot annot, this#type_ t_annot

  method type_annotation_hint (return: ('M, 'T) Ast.Type.annotation_or_hint)
                                     : ('N, 'U) Ast.Type.annotation_or_hint =
    let open Ast.Type in
    match return with
    | Available annot -> Available (this#type_annotation annot)
    | Missing loc -> Missing (this#on_type_annot loc)

  method function_ (expr: ('M, 'T) Ast.Function.t) : ('N, 'U) Ast.Function.t =
    let open Ast.Function in
    let {
      id = ident; params; body; async; generator;
      predicate; return; tparams; sig_loc;
    } = expr in
    let ident' = Option.map ~f:this#t_function_identifier ident in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let params' =
        let annot, { Params.params = params_list; rest } = params in
        let params_list' = Core_list.map ~f:this#function_param params_list in
        let rest' = Option.map ~f:this#function_rest_param rest in
        this#on_loc_annot annot, { Params.params = params_list'; rest = rest' }
      in
      let return' = this#type_annotation_hint return in
      let body' = match body with
        | BodyBlock (annot, block) ->
          BodyBlock (this#on_loc_annot annot, this#function_body block)
        | BodyExpression expr ->
          BodyExpression (this#expression expr)
      in
      let predicate' = Option.map ~f:this#type_predicate predicate in
      let sig_loc' = this#on_loc_annot sig_loc in
      {
        id = ident'; params = params'; return = return'; body = body';
        async; generator; predicate = predicate'; tparams = tparams'; sig_loc = sig_loc'
      }
    )

  method function_param (param: ('M, 'T) Ast.Function.Param.t) : ('N, 'U) Ast.Function.Param.t =
    let open Ast.Function.Param in
    let annot, { argument; default } = param in
    let annot' = this#on_loc_annot annot in
    let argument' = this#function_param_pattern argument in
    let default' = Option.map ~f:this#expression default in
    annot', { argument = argument'; default = default' }

  method function_rest_param (expr: ('M, 'T) Ast.Function.RestParam.t)
                                  : ('N, 'U) Ast.Function.RestParam.t =
    let open Ast.Function.RestParam in
    let annot, { argument } = expr in
    this#on_loc_annot annot, { argument = this#function_param_pattern argument }

  method function_body (block: ('M, 'T) Ast.Statement.Block.t) : ('N, 'U) Ast.Statement.Block.t =
    this#block block

  method function_identifier (ident: 'M Ast.Identifier.t) : 'N Ast.Identifier.t =
    this#pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Var ident

  method t_function_identifier (ident: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#t_pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Var ident

  method function_declaration (stmt: ('M, 'T) Ast.Function.t) : ('N, 'U) Ast.Function.t =
    this#function_ stmt

  method generator (expr: ('M, 'T) Ast.Expression.Generator.t)
                        : ('N, 'U) Ast.Expression.Generator.t =
    let open Ast.Expression.Generator in
    let { blocks; filter } = expr in
    let blocks' = Core_list.map ~f:this#comprehension_block blocks in
    let filter' = Option.map ~f:this#expression filter in
    { blocks = blocks'; filter = filter' }

  method identifier ((annot, name): 'M Ast.Identifier.t) : 'N Ast.Identifier.t =
    this#on_loc_annot annot, name

  method t_identifier ((annot, name): 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#on_type_annot annot, name

  method interface (interface: ('M, 'T) Ast.Statement.Interface.t)
                            : ('N, 'U) Ast.Statement.Interface.t =
    let open Ast.Statement.Interface in
    let { id = ident; tparams; extends; body } = interface in
    let id' = this#class_identifier ident in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let extends' = Core_list.map ~f:(this#on_loc_annot * this#generic_type) extends in
      let body' = (this#on_loc_annot * this#object_type) body in
      { id = id'; tparams = tparams'; extends = extends'; body = body' }
    )

  method interface_declaration (decl: ('M, 'T) Ast.Statement.Interface.t)
                                    : ('N, 'U) Ast.Statement.Interface.t =
    this#interface decl

  method private_name ((annot, ident): 'M Ast.PrivateName.t) : 'N Ast.PrivateName.t =
    this#on_loc_annot annot, this#identifier ident

  method import _annot (expr: ('M, 'T) Ast.Expression.t) : ('N, 'U) Ast.Expression.t =
    this#expression expr

  method if_consequent_statement ~has_else (stmt: ('M, 'T) Ast.Statement.t)
                                                : ('N, 'U) Ast.Statement.t =
    ignore has_else;
    this#statement stmt

  method if_statement (stmt: ('M, 'T) Ast.Statement.If.t) : ('N, 'U) Ast.Statement.If.t =
    let open Ast.Statement.If in
    let { test; consequent; alternate } = stmt in
    let test' = this#predicate_expression test in
    let consequent' =
      this#if_consequent_statement ~has_else:(alternate <> None) consequent in
    let alternate' = Option.map ~f:this#statement alternate in
    { test = test'; consequent = consequent'; alternate = alternate' }

  method import_declaration _loc (decl: ('M, 'T) Ast.Statement.ImportDeclaration.t)
                                      : ('N, 'U) Ast.Statement.ImportDeclaration.t =
    let open Ast.Statement.ImportDeclaration in
    let { importKind; source; specifiers; default } = decl in
    let specifiers' = Option.map ~f:this#import_specifier specifiers in
    let default' = Option.map ~f:this#import_default_specifier default in
    let source' = (this#on_loc_annot * id) source in
    { importKind; source = source'; specifiers = specifiers'; default = default' }

  method import_specifier (specifier: ('M, 'T) Ast.Statement.ImportDeclaration.specifier)
                                    : ('N, 'U) Ast.Statement.ImportDeclaration.specifier =
    let open Ast.Statement.ImportDeclaration in
    match specifier with
    | ImportNamedSpecifiers named_specifiers ->
      let named_specifiers' = Core_list.map ~f:this#import_named_specifier named_specifiers in
      ImportNamedSpecifiers named_specifiers'
    | ImportNamespaceSpecifier (annot, ident) ->
      let ident' = this#import_namespace_specifier ident in
      ImportNamespaceSpecifier (this#on_loc_annot annot, ident')

  method import_named_specifier (specifier: 'T Ast.Statement.ImportDeclaration.named_specifier)
                                          : 'U Ast.Statement.ImportDeclaration.named_specifier =
    let open Ast.Statement.ImportDeclaration in
    let { kind; local; remote } = specifier in
    let local' = Option.map ~f:this#t_pattern_identifier local in
    let remote' = this#t_pattern_identifier remote in
    { kind; local = local'; remote = remote' }

  method import_default_specifier (id: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#t_pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Let id

  method import_namespace_specifier (id: 'M Ast.Identifier.t) : 'N Ast.Identifier.t =
    this#pattern_identifier ~kind:Ast.Statement.VariableDeclaration.Let id

  method jsx_element (expr: ('M, 'T) Ast.JSX.element) =
    let open Ast.JSX in
    let { openingElement; closingElement; children } = expr in
    let openingElement' = this#jsx_opening_element openingElement in
    let closingElement' = Option.map ~f:this#jsx_closing_element closingElement in
    let children' = Core_list.map ~f:this#jsx_child children in
    { openingElement = openingElement'; closingElement = closingElement'; children = children' }

  method jsx_fragment (expr: ('M, 'T) Ast.JSX.fragment) : ('N, 'U) Ast.JSX.fragment =
    let open Ast.JSX in
    let { frag_openingElement; frag_closingElement; frag_children } = expr in
    let opening' = this#on_loc_annot frag_openingElement in
    let closing' = this#on_loc_annot frag_closingElement in
    let children' = Core_list.map ~f:this#jsx_child frag_children in
    { frag_openingElement = opening'; frag_closingElement = closing'; frag_children = children' }

  method jsx_opening_element (elem: ('M, 'T) Ast.JSX.Opening.t) : ('N, 'U) Ast.JSX.Opening.t =
    let open Ast.JSX.Opening in
    let annot, { name; selfClosing; attributes } = elem in
    let name' = this#jsx_name name in
    let attributes' = Core_list.map ~f:this#jsx_opening_attribute attributes in
    this#on_loc_annot annot, { name = name'; selfClosing; attributes = attributes' }

  method jsx_closing_element (elem: ('M, 'T) Ast.JSX.Closing.t) : ('N, 'U) Ast.JSX.Closing.t =
    let open Ast.JSX.Closing in
    let annot, {name} = elem in
    let name' = this#jsx_name name in
    this#on_loc_annot annot, {name=name'}

  method jsx_opening_attribute (jsx_attr: ('M, 'T) Ast.JSX.Opening.attribute)
                                        : ('N, 'U) Ast.JSX.Opening.attribute =
    let open Ast.JSX.Opening in
    match jsx_attr with
    | Attribute attr ->
      Attribute (this#jsx_attribute attr)
    | SpreadAttribute (annot, attr) ->
      SpreadAttribute (this#on_loc_annot annot, this#jsx_spread_attribute attr)

  method jsx_spread_attribute (attr: ('M, 'T) Ast.JSX.SpreadAttribute.t')
                                  : ('N, 'U) Ast.JSX.SpreadAttribute.t' =
    let open Ast.JSX.SpreadAttribute in
    let { argument } = attr in
    { argument = this#expression argument }

  method jsx_attribute (attr: ('M, 'T) Ast.JSX.Attribute.t) : ('N, 'U) Ast.JSX.Attribute.t =
    let open Ast.JSX.Attribute in
    let annot, { name; value } = attr in
    let name' = match name with
      | Identifier id -> Identifier (this#jsx_identifier id)
      | NamespacedName nname -> NamespacedName (this#jsx_namespaced_name nname)
    in
    let value' = Option.map ~f:this#jsx_attribute_value value in
    this#on_loc_annot annot, { name = name'; value = value' }

  method jsx_attribute_value (value: ('M, 'T) Ast.JSX.Attribute.value)
                                   : ('N, 'U) Ast.JSX.Attribute.value =
    let open Ast.JSX.Attribute in
    match value with
    | Literal (annot, lit) -> Literal (this#on_type_annot annot, lit)
    | ExpressionContainer (annot, expr) ->
      ExpressionContainer (this#on_type_annot annot, this#jsx_expression expr)

  method jsx_child (child: ('M, 'T) Ast.JSX.child) : ('N, 'U) Ast.JSX.child =
    let open Ast.JSX in
    let annot, child' = child in
    this#on_loc_annot annot,
    match child' with
    | Element elem ->
      Element (this#jsx_element elem)
    | Fragment frag ->
      Fragment (this#jsx_fragment frag)
    | ExpressionContainer expr ->
      ExpressionContainer (this#jsx_expression expr)
    | SpreadChild expr ->
      SpreadChild (this#expression expr)
    | Text _ as child' -> child'

  method jsx_expression (jsx_expr: ('M, 'T) Ast.JSX.ExpressionContainer.t)
                                 : ('N, 'U) Ast.JSX.ExpressionContainer.t =
    let open Ast.JSX.ExpressionContainer in
    let { expression } = jsx_expr in
    let expression' = match expression with
      | Expression expr -> Expression (this#expression expr)
      | EmptyExpression annot -> EmptyExpression (this#on_loc_annot annot)
    in
    { expression = expression' }

  method jsx_name (name: ('M, 'T) Ast.JSX.name): ('N, 'U) Ast.JSX.name =
    let open Ast.JSX in
    match name with
    | Identifier id -> Identifier (this#jsx_identifier id)
    | NamespacedName namespaced_name ->
      NamespacedName (this#jsx_namespaced_name namespaced_name)
    | MemberExpression member_exp ->
      MemberExpression (this#jsx_member_expression member_exp)

  method jsx_namespaced_name (namespaced_name: ('M, 'T) Ast.JSX.NamespacedName.t)
                                             : ('N, 'U) Ast.JSX.NamespacedName.t =
    let open Ast.JSX in
    let open NamespacedName in
    let annot, {namespace; name} = namespaced_name in
    let namespace' = this#jsx_identifier namespace in
    let name' = this#jsx_identifier name in
    this#on_loc_annot annot, {namespace=namespace'; name=name'}

  method jsx_member_expression (member_exp: ('M, 'T) Ast.JSX.MemberExpression.t)
                                          : ('N, 'U) Ast.JSX.MemberExpression.t =
    let open Ast.JSX in
    let annot, {MemberExpression._object; MemberExpression.property} = member_exp in
    let _object' = match _object with
      | MemberExpression.Identifier id ->
        let id' = this#jsx_identifier id in
        MemberExpression.Identifier id'
      | MemberExpression.MemberExpression nested_exp ->
        let nested_exp' = this#jsx_member_expression nested_exp in
        MemberExpression.MemberExpression nested_exp'
    in
    let property' = this#jsx_identifier property in
    this#on_loc_annot annot, MemberExpression.({_object=_object'; property=property'})

  method jsx_identifier ((annot, name): 'T Ast.JSX.Identifier.t) : 'U Ast.JSX.Identifier.t =
    this#on_type_annot annot, name

  method labeled_statement (stmt: ('M, 'T) Ast.Statement.Labeled.t)
                                : ('N, 'U) Ast.Statement.Labeled.t =
    let open Ast.Statement.Labeled in
    let { label; body } = stmt in
    let label' = this#label_identifier label in
    let body' = this#statement body in
    { label = label'; body = body' }

  method literal (expr: Ast.Literal.t) : Ast.Literal.t = expr

  method logical (expr: ('M, 'T) Ast.Expression.Logical.t) : ('N, 'U) Ast.Expression.Logical.t =
    let open Ast.Expression.Logical in
    let { operator; left; right } = expr in
    let left' = this#expression left in
    let right' = this#expression right in
    { operator; left = left'; right = right' }

  method member (expr: ('M, 'T) Ast.Expression.Member.t) : ('N, 'U) Ast.Expression.Member.t =
    let open Ast.Expression.Member in
    let { _object; property } = expr in
    let _object' = this#expression _object in
    let property' = this#member_property property in
    { _object = _object'; property = property' }

  method optional_member (expr: ('M, 'T) Ast.Expression.OptionalMember.t)
                              : ('N, 'U) Ast.Expression.OptionalMember.t =
    let open Ast.Expression.OptionalMember in
    let { member; optional } = expr in
    let member' = this#member member in
    { member = member'; optional }

  method member_property (expr: ('M, 'T) Ast.Expression.Member.property)
                              : ('N, 'U) Ast.Expression.Member.property =
    let open Ast.Expression.Member in
    match expr with
    | PropertyIdentifier ident ->
      PropertyIdentifier (this#member_property_identifier ident)
    | PropertyPrivateName ident ->
      PropertyPrivateName (this#member_private_name ident)
    | PropertyExpression e ->
      PropertyExpression (this#member_property_expression e)

  method member_property_identifier (ident: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#t_identifier ident

  method member_private_name (name: 'M Ast.PrivateName.t) : 'N Ast.PrivateName.t =
    this#private_name name

  method member_property_expression (expr: ('M, 'T) Ast.Expression.t) : ('N, 'U) Ast.Expression.t =
    this#expression expr

  method meta_property (expr: 'M Ast.Expression.MetaProperty.t)
                            : 'N Ast.Expression.MetaProperty.t =
    let open Ast.Expression.MetaProperty in
    let { meta; property } = expr in
    { meta = (this#on_loc_annot * id) meta; property = (this#on_loc_annot * id) property }

  method new_ (expr: ('M, 'T) Ast.Expression.New.t) : ('N, 'U) Ast.Expression.New.t =
    let open Ast.Expression.New in
    let { callee; targs; arguments } = expr in
    let callee' = this#expression callee in
    let targs' = Option.map ~f:this#type_parameter_instantiation_with_implicit targs in
    let arguments' = Core_list.map ~f:this#expression_or_spread arguments in
    { callee = callee'; targs = targs'; arguments = arguments' }

  method object_ (expr: ('M, 'T) Ast.Expression.Object.t) : ('N, 'U) Ast.Expression.Object.t =
    let open Ast.Expression.Object in
    let { properties } = expr in
    let properties' = Core_list.map ~f:(fun prop ->
      match prop with
      | Property p ->
        let p' = this#object_property p in
        Property p'
      | SpreadProperty s ->
        let s' = this#spread_property s in
        SpreadProperty s'
    ) properties in
    { properties = properties' }

  method object_property (prop: ('M, 'T) Ast.Expression.Object.Property.t)
                              : ('N, 'U) Ast.Expression.Object.Property.t =
    let open Ast.Expression.Object.Property in
    let annot, prop' = prop in
    this#on_loc_annot annot,
    match prop' with
    | Init { key; value; shorthand } ->
      let key' = this#object_key key in
      let value' = this#expression value in
      Init { key = key'; value = value'; shorthand }

    | Method { key; value = fn_annot, fn } ->
      let key' = this#object_key key in
      let fn' = this#function_ fn in
      Method { key = key'; value = this#on_loc_annot fn_annot, fn' }

    | Get { key; value = fn_annot, fn } ->
      let key' = this#object_key key in
      let fn' = this#function_ fn in
      Get { key = key'; value = this#on_loc_annot fn_annot, fn' }

    | Set { key; value = fn_annot, fn } ->
      let key' = this#object_key key in
      let fn' = this#function_ fn in
      Set { key = key'; value = this#on_loc_annot fn_annot, fn' }

  method object_key (key: ('M, 'T) Ast.Expression.Object.Property.key) =
    let open Ast.Expression.Object.Property in
    match key with
    | Literal (annot, lit) ->
      Literal (this#on_type_annot annot, this#literal lit)
    | Identifier ident ->
      Identifier (this#object_key_identifier ident)
    | PrivateName ident ->
      PrivateName (this#private_name ident)
    | Computed expr ->
      Computed (this#expression expr)

  method object_key_identifier (ident: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    this#t_identifier ident

  method opaque_type (otype: ('M, 'T) Ast.Statement.OpaqueType.t)
                          : ('N, 'U) Ast.Statement.OpaqueType.t =
    let open Ast.Statement.OpaqueType in
    let { id; tparams; impltype; supertype } = otype in
    let id' = this#t_identifier id in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let impltype' = Option.map ~f:this#type_ impltype in
      let supertype' = Option.map ~f:this#type_ supertype  in
      {
        id = id';
        tparams = tparams';
        impltype = impltype';
        supertype = supertype'
      }
    )

  method declare_opaque_type (otype: ('M, 'T) Ast.Statement.OpaqueType.t)
                                  : ('N, 'U) Ast.Statement.OpaqueType.t =
    this#opaque_type otype

  method function_param_pattern (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#binding_pattern expr

  method variable_declarator_pattern ~kind (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#binding_pattern ~kind expr

  method catch_clause_pattern (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#binding_pattern ~kind:Ast.Statement.VariableDeclaration.Let expr

  method for_in_assignment_pattern (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#assignment_pattern expr

  method for_of_assignment_pattern (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#assignment_pattern expr

  method binding_pattern
    ?(kind=Ast.Statement.VariableDeclaration.Var)
    (expr: ('M, 'T) Ast.Pattern.t)
    : ('N, 'U) Ast.Pattern.t =
    this#pattern ~kind expr

  method assignment_pattern (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#pattern expr

  (* NOTE: Patterns are highly overloaded. A pattern can be a binding pattern,
     which has a kind (Var/Let/Const, with Var being the default for all pre-ES5
     bindings), or an assignment pattern, which has no kind. Subterms that are
     patterns inherit the kind (or lack thereof). *)
  method pattern ?kind (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    let open Ast.Pattern in
    let annot, patt = expr in
    this#on_type_annot annot,
    match patt with
    | Object { Object.properties; annot } ->
      let properties' = Core_list.map ~f:(this#pattern_object_p ?kind) properties in
      let annot' = this#type_annotation_hint annot in
      Object { Object.properties = properties'; annot = annot' }
    | Array { Array.elements; annot } ->
      let elements' = Core_list.map ~f:(Option.map ~f:(this#pattern_array_e ?kind)) elements in
      let annot' = this#type_annotation_hint annot in
      Array { Array.elements = elements'; annot = annot' }
    | Identifier { Identifier.name; annot; optional } ->
      let name' = this#t_pattern_identifier ?kind name in
      let annot' = this#type_annotation_hint annot in
      Identifier { Identifier.name = name'; annot = annot'; optional }
    | Expression e ->
      Expression (this#pattern_expression e)

  method pattern_identifier ?kind (ident: 'M Ast.Identifier.t) : 'N Ast.Identifier.t =
    ignore kind;
    this#identifier ident

  method t_pattern_identifier ?kind (ident: 'T Ast.Identifier.t) : 'U Ast.Identifier.t =
    ignore kind;
    this#t_identifier ident

  method pattern_literal ?kind (expr: Ast.Literal.t) : Ast.Literal.t =
    ignore kind;
    this#literal expr

  method pattern_object_p ?kind (p: ('M, 'T) Ast.Pattern.Object.property) =
    let open Ast.Pattern.Object in
    match p with
    | Property (annot, prop) ->
      Property (this#on_loc_annot annot, this#pattern_object_property ?kind prop)
    | RestProperty (annot, prop) ->
      RestProperty (this#on_loc_annot annot, this#pattern_object_rest_property ?kind prop)

  method pattern_object_property ?kind (prop: ('M, 'T) Ast.Pattern.Object.Property.t')
                                            : ('N, 'U) Ast.Pattern.Object.Property.t' =
    let open Ast.Pattern.Object.Property in
    let { key; pattern; default; shorthand } = prop in
    let key' = this#pattern_object_property_key ?kind key in
    let pattern' = this#pattern_object_property_pattern ?kind pattern in
    let default' = Option.map ~f:this#expression default in
    { key = key'; pattern = pattern'; default = default'; shorthand }

  method pattern_object_property_key ?kind (key: ('M, 'T) Ast.Pattern.Object.Property.key) =
    let open Ast.Pattern.Object.Property in
    match key with
    | Literal (annot, lit) ->
      Literal (this#on_loc_annot annot, this#pattern_object_property_literal_key ?kind lit)
    | Identifier identifier ->
      Identifier (this#pattern_object_property_identifier_key ?kind identifier)
    | Computed expr ->
      Computed (this#pattern_object_property_computed_key ?kind expr)

  method pattern_object_property_literal_key ?kind (key: Ast.Literal.t) : Ast.Literal.t =
    this#pattern_literal ?kind key

  method pattern_object_property_identifier_key ?kind (key: 'M Ast.Identifier.t)
                                                          : 'N Ast.Identifier.t =
    this#pattern_identifier ?kind key

  method pattern_object_property_computed_key ?kind (key: ('M, 'T) Ast.Expression.t)
                                                        : ('N, 'U) Ast.Expression.t =
    ignore kind;
    this#pattern_expression key

  method pattern_object_rest_property ?kind (prop: ('M, 'T) Ast.Pattern.Object.RestProperty.t')
                                                : ('N, 'U) Ast.Pattern.Object.RestProperty.t' =
    let open Ast.Pattern.Object.RestProperty in
    let { argument } = prop in
    let argument' = this#pattern_object_rest_property_pattern ?kind argument in
    { argument = argument' }

  method pattern_object_property_pattern ?kind (expr: ('M, 'T) Ast.Pattern.t)
                                                    : ('N, 'U) Ast.Pattern.t =
    this#pattern ?kind expr

  method pattern_object_rest_property_pattern ?kind (expr: ('M, 'T) Ast.Pattern.t)
                                                        : ('N, 'U) Ast.Pattern.t =
    this#pattern ?kind expr

  method pattern_array_e ?kind (e: ('M, 'T) Ast.Pattern.Array.element)
                                : ('N, 'U) Ast.Pattern.Array.element =
    let open Ast.Pattern.Array in
    match e with
    | Element (annot, elem) ->
      Element (this#on_loc_annot annot, this#pattern_array_element ?kind elem)
    | RestElement (annot, elem) ->
      RestElement (this#on_loc_annot annot, this#pattern_array_rest_element ?kind elem)

  method pattern_array_element ?kind (elem: ('M, 'T) Ast.Pattern.Array.Element.t')
                                          : ('N, 'U) Ast.Pattern.Array.Element.t' =
    let open Ast.Pattern.Array.Element in
    let { argument; default } = elem in
    let argument' = this#pattern_array_element_pattern ?kind argument in
    let default' = Option.map ~f:this#expression default in
    { argument = argument'; default = default' }

  method pattern_array_element_pattern ?kind (expr: ('M, 'T) Ast.Pattern.t)
                                                  : ('N, 'U) Ast.Pattern.t =
    this#pattern ?kind expr

  method pattern_array_rest_element ?kind (elem: ('M, 'T) Ast.Pattern.Array.RestElement.t') =
    let open Ast.Pattern.Array.RestElement in
    let { argument } = elem in
    let argument' = this#pattern_array_rest_element_pattern ?kind argument in
    { argument = argument' }

  method pattern_array_rest_element_pattern ?kind (expr: ('M, 'T) Ast.Pattern.t)
                                                      : ('N, 'U) Ast.Pattern.t =
    this#pattern ?kind expr

  method pattern_assignment_pattern ?kind (expr: ('M, 'T) Ast.Pattern.t) : ('N, 'U) Ast.Pattern.t =
    this#pattern ?kind expr

  method pattern_expression (expr: ('M, 'T) Ast.Expression.t) : ('N, 'U) Ast.Expression.t =
    this#expression expr

  method predicate_expression (expr: ('M, 'T) Ast.Expression.t) : ('N, 'U) Ast.Expression.t =
    this#expression expr

  method return (stmt: ('M, 'T) Ast.Statement.Return.t) : ('N, 'U) Ast.Statement.Return.t =
    let open Ast.Statement.Return in
    let { argument } = stmt in
    let argument' = Option.map ~f:this#expression argument in
    { argument = argument' }

  method sequence (expr: ('M, 'T) Ast.Expression.Sequence.t) : ('N, 'U) Ast.Expression.Sequence.t =
    let open Ast.Expression.Sequence in
    let { expressions } = expr in
    let expressions' = Core_list.map ~f:this#expression expressions in
    { expressions = expressions' }

  method toplevel_statement_list (stmts: ('M, 'T) Ast.Statement.t list)
                                      : ('N, 'U) Ast.Statement.t list =
    this#statement_list stmts

  method statement_list (stmts: ('M, 'T) Ast.Statement.t list) : ('N, 'U) Ast.Statement.t list =
    Core_list.map ~f:this#statement stmts

  method spread_element (expr: ('M, 'T) Ast.Expression.SpreadElement.t)
                            : ('N, 'U) Ast.Expression.SpreadElement.t =
    let open Ast.Expression.SpreadElement in
    let annot, { argument } = expr in
    this#on_loc_annot annot, { argument = this#expression argument }

  method spread_property (expr: ('M, 'T) Ast.Expression.Object.SpreadProperty.t)
                              : ('N, 'U) Ast.Expression.Object.SpreadProperty.t =
    let open Ast.Expression.Object.SpreadProperty in
    let annot, { argument } = expr in
    this#on_loc_annot annot, { argument = this#expression argument }

  method switch (switch: ('M, 'T) Ast.Statement.Switch.t) : ('N, 'U) Ast.Statement.Switch.t =
    let open Ast.Statement.Switch in
    let { discriminant; cases } = switch in
    let discriminant' = this#expression discriminant in
    let cases' = Core_list.map ~f:(this#on_loc_annot * this#switch_case) cases in
    { discriminant = discriminant'; cases = cases' }

  method switch_case (case: ('M, 'T) Ast.Statement.Switch.Case.t') =
    let open Ast.Statement.Switch.Case in
    let { test; consequent } = case in
    let test' = Option.map ~f:this#expression test in
    let consequent' = this#statement_list consequent in
    { test = test'; consequent = consequent' }

  method tagged_template (expr: ('M, 'T) Ast.Expression.TaggedTemplate.t)
                              : ('N, 'U) Ast.Expression.TaggedTemplate.t =
    let open Ast.Expression.TaggedTemplate in
    let { tag; quasi } = expr in
    let tag' = this#expression tag in
    let quasi' = (this#on_loc_annot * this#template_literal) quasi in
    { tag = tag'; quasi = quasi' }

  method template_literal (expr: ('M, 'T) Ast.Expression.TemplateLiteral.t)
                              : ('N, 'U) Ast.Expression.TemplateLiteral.t =
    let open Ast.Expression.TemplateLiteral in
    let { quasis; expressions } = expr in
    let quasis' = Core_list.map ~f:this#template_literal_element quasis in
    let expressions' = Core_list.map ~f:this#expression expressions in
    { quasis = quasis'; expressions = expressions' }

  method template_literal_element ((annot, elem): 'M Ast.Expression.TemplateLiteral.Element.t)
                                                : 'N Ast.Expression.TemplateLiteral.Element.t =
    this#on_loc_annot annot, elem

  method throw (stmt: ('M, 'T) Ast.Statement.Throw.t) : ('N, 'U) Ast.Statement.Throw.t =
    let open Ast.Statement.Throw in
    let { argument } = stmt in
    { argument = this#expression argument }

  method try_catch (stmt: ('M, 'T) Ast.Statement.Try.t) : ('N, 'U) Ast.Statement.Try.t =
    let open Ast.Statement.Try in
    let { block; handler; finalizer } = stmt in
    let block' = (this#on_loc_annot * this#block) block in
    let handler' = Option.map ~f:(this#on_loc_annot * this#catch_clause) handler in
    let finalizer' = Option.map ~f:(this#on_loc_annot * this#block) finalizer in
    {
      block = block';
      handler = handler';
      finalizer = finalizer'
    }

  method type_cast (expr: ('M, 'T) Ast.Expression.TypeCast.t) : ('N, 'U) Ast.Expression.TypeCast.t =
    let open Ast.Expression.TypeCast in
    let { expression; annot; } = expr in
    let expression' = this#expression expression in
    let annot' = this#type_annotation annot in
    { expression = expression'; annot = annot' }

  method unary_expression (expr: ('M, 'T) Ast.Expression.Unary.t)
                              : ('N, 'U) Ast.Expression.Unary.t =
    let open Ast.Expression.Unary in
    let { argument; operator; } = expr in
    let argument' = this#expression argument in
    { argument = argument'; operator }

  method update_expression (expr: ('M, 'T) Ast.Expression.Update.t)
                                : ('N, 'U) Ast.Expression.Update.t =
    let open Ast.Expression.Update in
    let { argument; operator; prefix; } = expr in
    let argument' = this#expression argument in
    { argument = argument'; operator; prefix }

  method variable_declaration (decl: ('M, 'T) Ast.Statement.VariableDeclaration.t)
                                  : ('N, 'U) Ast.Statement.VariableDeclaration.t =
    let open Ast.Statement.VariableDeclaration in
    let { declarations; kind } = decl in
    let decls' = Core_list.map ~f:(this#variable_declarator ~kind) declarations in
    { declarations = decls'; kind }

  method variable_declarator ~kind (decl: ('M, 'T) Ast.Statement.VariableDeclaration.Declarator.t)
                                        : ('N, 'U) Ast.Statement.VariableDeclaration.Declarator.t =
    let open Ast.Statement.VariableDeclaration.Declarator in
    let annot, { id; init } = decl in
    let id' = this#variable_declarator_pattern ~kind id in
    let init' = Option.map ~f:this#expression init in
    this#on_loc_annot annot, { id = id'; init = init' }

  method while_ (stuff: ('M, 'T) Ast.Statement.While.t) : ('N, 'U) Ast.Statement.While.t =
    let open Ast.Statement.While in
    let { test; body } = stuff in
    let test' = this#predicate_expression test in
    let body' = this#statement body in
    { test = test'; body = body' }

  method with_ (stuff: ('M, 'T) Ast.Statement.With.t) : ('N, 'U) Ast.Statement.With.t =
    let open Ast.Statement.With in
    let { _object; body } = stuff in
    let _object' = this#expression _object in
    let body' = this#statement body in
    { _object = _object'; body = body' }

  method type_alias (stuff: ('M, 'T) Ast.Statement.TypeAlias.t)
                          : ('N, 'U) Ast.Statement.TypeAlias.t =
    let open Ast.Statement.TypeAlias in
    let { id; tparams; right } = stuff in
    let id' = this#t_identifier id in
    this#type_parameter_declaration_opt tparams (fun tparams' ->
      let right' = this#type_ right in
      { id = id'; tparams = tparams'; right = right' }
    )

  method yield (expr: ('M, 'T) Ast.Expression.Yield.t) : ('N, 'U) Ast.Expression.Yield.t =
    let open Ast.Expression.Yield in
    let { argument; delegate } = expr in
    let argument' = Option.map ~f:this#expression argument in
    { argument = argument'; delegate }

end

let fold_program mappers ast =
  List.fold_left (fun ast m -> m#program ast) ast mappers
