# Built-in auxiliary type checkers

version 5.0.0

## 1. Introduction

Groovy’s static nature includes an extensible type-checking mechanism. This mechanism allows users to selectively strengthen or weaken type checking as needed to cater for scenarios where the standard type checking isn’t sufficient.

In addition to allowing you to write your own customer checkers, Groovy offers a handful of useful built-in type checkers. These provide additional checks for specific scenarios.

In the examples which follow, we’ll explicitly show declaring the type checker we want to use. As usual, Groovy’s compiler customization mechanisms would allow you to simplify application of such checkers, e.g. make it apply globally using a compiler configuration script, as just one example.

## 2. Checking Regular Expressions

A regular expression (regex) defines a search pattern for text. The pattern can be as simple as a single character, a fixed string, or a complex expression containing special characters describing a pattern. The JDK has special regex classes and Groovy adds some special syntactic sugar and functionality on top of the JDK classes.

Here is an example the checks that the String 'Foo' matches a regular expression:

    assert 'foo'.matches(/[Ff].{2}\b/)

The regex pattern is made up of three parts:

- `[Ff]` is a character class matching upper or lowercase 'f',

- `.{2}` means exactly two occurrences of the `.` character class which matches any character

- `\b` matches a word boundary

There are more options we could add here such as grouping, lookahead and look-behind, to name just a few. With so many options and special characters, it isn’t hard to create an invalid regular expression pattern. Such invalid patterns are typically found at runtime with a runtime exception.

The goal of the `RegexChecker` is to find such errors at compile-time. If compilation of the expressions succeeds, they are guaranteed not to fail at runtime.

### 2.1. Typical usage

The previous example can be type checked as follows:

    @TypeChecked(extensions='groovy.typecheckers.RegexChecker')
    static main(args) {
        assert 'foo'.matches(/[Ff].{2}\b/)
    }

If you have many examples where such checking is desirable, you could consider automatically adding this extension to your compilation process.

### 2.2. Covered methods

The methods which are checked include:

`java.util.regex.Pattern`
    static Pattern compile(String regex)
    static Pattern compile(String regex, int flags)
    static boolean matches(String regex, CharSequence input)
`java.util.regex.Matcher`
    String group(int group)
    matcher[matchCount][groupCount]  // getAt shorthand
Operators
    ~/regex/                 // pattern operator
    'string' =~ /regex/      // find operator
    'string' ==~ /regex/     // (exact) match operator

The RegexChecker will check any method calls listed above. It might find such methods when they involve explicit static method calls or when the receiver’s type can be inferred.

The regular expression must be a constant-like String. Any group-count values must be a constant-like integer. By constant-like, we mean, either an explicit String literal, or a field with an initializer expression or a local variable with an initializer expression. These possibilities are shown in the following code, which contains 3 regular expressions:

    static final String TWO_GROUPS_OF_THREE = /(...)(...)/  (1)

    @TypeChecked(extensions='groovy.typecheckers.RegexChecker')
    static main(args) {
        var m = 'foobar' =~ TWO_GROUPS_OF_THREE  //  (2)
        assert m.find()
        assert m.group(1) == 'foo'
        assert m.group(2) == 'bar'  (3)

        var pets = ['cat', 'dog', 'goldfish']
        var shortNamed = ~/\w{3}/  (4)
        assert pets.grep(shortNamed) == ['cat', 'dog']  (5)

        assert Pattern.matches(/(\d{4})-(\d{1,2})-(\d{1,2})/, '2020-12-31')  (6)
    }

1 | A constant String regex
2 | Using the constant regex with the find operator
3 | Checking the matcher’s second group
4 | A local variable containing a regex pattern
5 | Using the pattern with grep
6 | An API call passing a String literal regex

### 2.3. Errors detected

Luckily, in the above example, we made no errors in our regex definitions, and the code compiles and executes successfully.

If however, we did make certain errors in those definitions, then we would expect the potential of runtime errors. Using `RegexChecker`, we can find certain kinds of errors during compilation. Let’s look at some examples.

Suppose at ❸ we looked for the third group instead of the second:

    assert m.group(3) == 'bar'

We would see the following error at compile-time:

    [Static type checking] - Invalid group count 3 for regex with 2 groups

Alternatively, suppose at ❹ we accidentally left off the closing curly brace:

    var shortNamed = ~/\w{3/

We would see the following error at compile-time:

    [Static type checking] - Bad regex: Unclosed counted closure near index 4

Alternatively, suppose at ❻ we accidentally left off the closing round bracket for the last (day of month) group:

    assert Pattern.matches(/(\d{4})-(\d{1,2})-(\d{1,2}/, '2020-12-31')

We would see the following error at compile-time:

    [Static type checking] - Bad regex: Unclosed group near index 26

Over-and-above these examples, detected errors include:

- Bad class syntax

- Bad named capturing group

- Dangling meta character

- Empty character family

- Illegal character range

- Illegal repetition

- Illegal/unsupported escape sequence

- Look-behind group does not have an obvious maximum length

- Unexpected character

- Unclosed character family

- Unclosed character class

- Unclosed counted closure

- Unclosed group

- Unknown character property name

- Unknown Unicode property

- Unknown group type

- Unknown inline modifier

- Unknown character name

- Unmatched closing ')'

## 3. Checking Format Strings

The `format` methods in `java.util.Formatter`, and other similar methods, support formatted printing in the style of C’s `printf` method with a format string and zero or more arguments.

Let’s consider an example which produces a string comprised of three terms:

- a floating-point representation (`%f`) of PI (with 2 decimal places of precision),

- the hex representation (`%X`) of 15 (as two uppercase digits with a leading 0),

- and the Boolean (`%B`) `True` (in uppercase).

The assertion checks our expectations:

    assert String.format('%4.2f %02X %B', Math.PI, 15, true) == '3.14 0F TRUE'

This is a powerful method supporting numerous conversions and flags. If the developer supplies incorrect conversions or flags, they will receive one of numerous possible runtime errors. As examples, consider the following mistakes and resulting runtime exceptions:

- supplying a String as the parameter for either of the first two arguments results in an `IllegalFormatConversionException`,

- leaving out the last argument results in a `MissingFormatArgumentException`,

- supplying the leading zero flag for the Boolean parameter results in a `FlagsConversionMismatchException`.

The goal of the `FormatStringChecker` is to eliminate a large number of such runtime errors. If the API call passes type checking, it will be guaranteed to succeed at runtime.

### 3.1. Typical usage

Here is an example of correctly using some of these methods with type checking in place:

    @TypeChecked(extensions='groovy.typecheckers.FormatStringChecker')
    static main(args) {
        assert String.format('%x', 1023) == '3ff'
        assert String.format(Locale.US, '%x', 1024) == '400'
        assert sprintf('%s%s', 'foo', 'bar') == 'foobar'
        assert new Formatter().format('xy%s', 'zzy').toString() == 'xyzzy'
        def baos = new ByteArrayOutputStream()
        new PrintStream(baos).printf('%-4c|%6b', 'x' as char, true)
        assert baos.toString() == 'x   |  true'
    }

Over and above these methods, if you have your own method of this style, you can annotate it with `@FormatMethod`, and it will also be checked.

    @FormatMethod
    static String log(String formatString, Object... args) {
        sprintf(formatString, args)
    }

    @TypeChecked(extensions='groovy.typecheckers.FormatStringChecker')
    static main(args) {
        assert log('%x', 1023) == '3ff'
        assert log('%x', 1024) == '400'
    }

You can use the `FormatMethod` annotation provided in the `groovy-typecheckers` module or the similarly named annotation from the Java-based checker framework.

### 3.2. Covered methods

The format string methods have the following characteristics:

- The first argument must be of type `java.util.Locale` or `java.lang.String`.

- If the first argument is of type `Locale`, the second argument must be of type `String`.

- The `String` argument is treated as a format string containing zero or more embedded format specifiers as well as plain text. The format specifiers determine how the remaining arguments will be used within the resulting output.

The `FormatStringChecker` ensures that the format string is valid and the remaining arguments are compatible with the embedded format specifiers.

The methods which are checked include:

`java.lang.String`
    static String format(String format, Object... args)
    static String format(Locale l, String format, Object... args)
    String formatted(Object... args)  // JDK15+
`java.util.Formatter`
    String format(String format, Object... args)
    String format(Locale l, String format, Object... args)
`java.io.PrintStream`
    PrintStream printf(String format, Object... args)
    PrintStream printf(Locale l, String format, Object... args)
DGM methods
    printf(String format, Object... args)
    printf(Locale l, String format, Object... args)
    sprintf(String format, Object... args)
    sprintf(Locale l, String format, Object... args)

The format string checker looks for a constant literal format string. For arguments, it also looks for constant literals or otherwise makes checks based on inferred type. When looking for constants, the checker will find inline literals, local variables with a constant initializer, and fields with a constant initializer. Here are some examples:

    static final String FIELD_PATTERN = '%x'
    static final int FIELD_PARAM = 1023

    @TypeChecked(extensions='groovy.typecheckers.FormatStringChecker')
    static main(args) {
        assert sprintf('%x', 1024) == '400'  (1)

        assert sprintf(FIELD_PATTERN, FIELD_PARAM) == '3ff'  (2)

        var local_var_pattern = '%x'
        var local_var_param = 1022
        assert sprintf(local_var_pattern, local_var_param) == '3fe'  (3)

        assert sprintf('%x%s', theParam(), 'a' + 'b') == '3fdab'  (4)
    }

    static int theParam() { 1021 }

1 | Constant literals
2 | Fields with an initializer
3 | Local variables with an initializer
4 | Checking of the parameter will be based on inferred type

### 3.3. Errors detected

Here are some examples of the kinds of errors detected.

- The arguments much match the type of the format classifier, e.g. we can’t use the character classifier with a boolean argument:

      sprintf '%c', true

  We would see the following error at compile-time:

      [Static type checking] - IllegalFormatConversion: c != java.lang.Boolean

- We shouldn’t specify precision for non-scientific arguments, e.g. we shouldn’t expect 7 decimal places of precision for an integral value:

      String.format('%1.7d', 3)

  We would see the following error at compile-time:

      [Static type checking] - IllegalFormatPrecision: 7

- We should specify only known format specifier conversions, e.g. `v` isn’t known:

      System.out.printf('%v', 7)

  We would see the following error at compile-time:

      [Static type checking] - UnknownFormatConversion: Conversion = 'v'

The complete list of errors detected include:

- duplicate format flags

- illegal format flags

- illegal format precision values

- unknown format conversions

- illegal format conversions

- format flags conversion mismatches

- missing arguments