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# NAME

Syntax::Keyword::Assert - assert keyword for Perl with zero runtime cost

# SYNOPSIS

```perl
use Syntax::Keyword::Assert;

my $obj = bless {}, "Foo";
assert($obj isa "Bar");
# => Assertion failed (Foo=HASH(0x11e022818) isa "Bar")

assert($x > 0, "x must be positive");
# => x must be positive
```

# DESCRIPTION

Syntax::Keyword::Assert provides a syntax extension for Perl that introduces a `assert` keyword.

By default assertions are enabled, but can be disabled by setting `$ENV{PERL_ASSERT_ENABLED}` to false before this module is loaded:

```
BEGIN { $ENV{PERL_ASSERT_ENABLED} = 0 }  # Disable assertions
```

When assertions are disabled, the `assert` are completely ignored at compile phase, resulting in zero runtime cost. This makes Syntax::Keyword::Assert ideal for use in production environments, as it does not introduce any performance penalties when assertions are not needed.

# KEYWORDS

## assert

```
assert(EXPR)
assert(EXPR, MESSAGE)
```

If EXPR is truthy in scalar context, then happens nothing. Otherwise, it dies with a user-friendly error message.

Here are some examples:

```perl
assert("apple" eq "banana");  # => Assertion failed ("apple" eq "banana")
assert(123 != 123);           # => Assertion failed (123 != 123)
assert(1 > 10);               # => Assertion failed (1 > 10)
```

You can provide a custom error message as the second argument:

```perl
assert($x > 0, "x must be positive");
# => x must be positive
```

The message expression is lazily evaluated. It is only evaluated when the assertion fails.
This is equivalent to:

```
$cond || do { die $msg }
```

This means you can use expensive computations or side effects in the message without worrying about performance when the assertion passes:

```
assert($x > 0, expensive_debug_info());
# expensive_debug_info() is NOT called if $x > 0
```

# METHODS

## Importing

```perl
use Syntax::Keyword::Assert;
```

Importing the module enables the `assert` keyword in the current lexical scope (see ["Unimporting"](#unimporting) below).

`assert` is enabled per lexical scope via `import_into`, so you can build a "toolkit" module that re-exports it to your users, the same way [Object::Pad](https://metacpan.org/pod/Object%3A%3APad) or [Syntax::Keyword::Try](https://metacpan.org/pod/Syntax%3A%3AKeyword%3A%3ATry) do:

```perl
package MyToolkit;

use Syntax::Keyword::Assert ();

sub import {
    my $class  = shift;
    my $caller = caller;

    Syntax::Keyword::Assert->import_into( $caller );
    # ... enable other keywords/pragmas here too
}
```

Now `use MyToolkit;` gives the caller the `assert` keyword as well, without them having to `use Syntax::Keyword::Assert` directly.

## Unimporting

The `assert` keyword is lexically scoped. You can disable it for the rest of the enclosing scope with:

```perl
use Syntax::Keyword::Assert;

assert(1);  # ok

no Syntax::Keyword::Assert;

assert(1);  # => Undefined subroutine &main::assert called ...
```

Once unimported, `assert` is no longer recognised as a keyword; it is parsed as an ordinary function call instead, which dies at runtime unless a subroutine of that name exists.

# SEE ALSO

- [PerlX::Assert](https://metacpan.org/pod/PerlX%3A%3AAssert)

    This module also uses keyword plugin, but it depends on [Keyword::Simple](https://metacpan.org/pod/Keyword%3A%3ASimple). And this module's error message does not include the failed expression.

- [Devel::Assert](https://metacpan.org/pod/Devel%3A%3AAssert)

    This module provides a similar functionality, but it does not use a keyword plugin.

# LICENSE

Copyright (C) kobaken.

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

# AUTHOR

kobaken <kentafly88@gmail.com>