Type::Tiny and Smart Match

Perl 5.10 introduced the smart match operator "~~", which has since been deprecated because though the general idea is fairly sound, the details were a bit messy.

Nevertheless, Type::Tiny has support for smart match and I'm documenting it here because there's nowhere better to put it.

The following can be used as to check if a value passes a type constraint:

  $value ~~ SomeType

Where it gets weird is if $value is an object and overloads "~~". Which overload of "~~" wins? I don't know.

Better to use:

  SomeType->check( $value )   # more reliable, probably faster
  is_SomeType($value)         # more reliable, definitely faster

It's also possible to do:

  $value ~~ SomeType->coercion

This checks to see if $value matches any type that can be coerced to SomeType.

But better to use:

  SomeType->coercion->has_coercion_for_value( $value )

"given" and "when"

Related to the smart match operator is the "given"/"when" syntax.

This will not do what you want it to do:

  use Types::Standard qw( Str Int );
  
  given ($value) {
    when (Int) { ... }
    when (Str) { ... }
  }

This will do what you wanted:

  use Types::Standard qw( is_Str is_Int );
  
  given ($value) {
    when (\&is_Int) { ... }
    when (\&is_Str) { ... }
  }

Sorry, that's just how Perl be.

Better though:

  use Types::Standard qw( Str Int );
  use Type::Utils qw( match_on_type );
  
  match_on_type $value => (
    Str, sub { ... },
    Int, sub { ... },
  );

If this is part of a loop or other frequently called bit of code, you can compile the checks once and use them many times:

  use Types::Standard qw( Str Int );
  use Type::Utils qw( compile_match_on_type );
  
  my $dispatch_table = compile_match_on_type(
    Str, sub { ... },
    Int, sub { ... },
  );
  
  $dispatch_table->($_) for @lots_of_values;

As with most things in Type::Tiny, those coderefs can be replaced by strings of Perl code.