Common type constructs, done in Perl

Types.pm

package Direction;

# Simple algebraic types
sub North {
    die 'Invalid type' unless @_ == 1;
    bless(sub { 'North' }, shift)
}
sub South {
    die 'Invalid type' unless @_ == 1;
    bless(sub { 'South' }, shift)
}

package Int;

# Simple type with value constructor
sub Int {
    die 'Invalid type' unless @_ == 1;
    my $type = shift;
    return sub {
        my $value = shift;
        # Value constructor constraints go here
        die 'Invalid value' unless $value =~ /^\d+$/;
        bless(sub { $value }, $type);
    }
}

package List;

# Recursive, parameterized type

sub Empty {
    die 'Invalid type' unless @_ == 2;
    my $type = join('.', @_);

    bless(sub { 'Empty' }, $type);
}

sub Cons {
    die 'Invalid type' unless @_ == 2;
    my $type = join('.', @_);
    my $valuetype = $_[1];

    return sub {
        my $value = shift;

        return sub  {
            my $list = shift;

            # Value constructor constraints go here
            die unless $value->isa($valuetype);
            die unless $list->isa($type);
            return bless(sub { $value, $list }, $type);
        }
    }
}

1;


### EXAMPLE USAGE

# REPL:42> List->Empty('Int')
# bless( sub {
#     package List;
#     'Empty';
# }, 'List.Int' )

# REPL:43> List->Empty('Int')->()
# Empty
# REPL:44> List->Cons('Int')->(3)->(List->Empty('Int'))
# Runtime error: Can't call method "isa" without a package or object reference at Types.pm line 50.

# REPL:45> List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))
# bless( sub {
#     package List;
#     $value, $list;
# }, 'List.Int' )

# REPL:46> List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))->()
# [
#   bless( sub {
#       package Int;
#       $value;
#   }, 'Int' ),
#   bless( sub {
#       package List;
#       'Empty';
#   }, 'List.Int' )
# ]

# REPL:47> List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))->()[0]
# Runtime error: Not an ARRAY reference at (eval 2815) line 8.

# REPL:48> List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))->()
# [
#   bless( sub {
#       package Int;
#       $value;
#   }, 'Int' ),
#   bless( sub {
#       package List;
#       'Empty';
#   }, 'List.Int' )
# ]

# REPL:49> (List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))->())[0]
# bless( sub {
#     package Int;
#     $value;
# }, 'Int' )

# REPL:50> (List->Cons('Int')->(Int->Int->(3))->(List->Empty('Int'))->())[0]->()
# 3