Restricting a scalar to certain values

[Anonymous Monk]

Dear Monks,

In larger programs, I like to guard against typos in hash keys (when the keys are a known, fixed set) using Hash::Util's lock_keys, or I'll retrofit a large program with this module for the same purpose.

Is there something like this for scalars? I.e. something that lets me restrict the value of a scalar to a set of allowed values? (I've searched, but not found anything yet.) Something like this:

tie my $state, 'Scalar::Restrict', qw/ idle waiting active /;
# or maybe
lock_scalar my $state, qw/ idle waiting active /;

# $state may now only be "idle", "waiting", or "active"

Thanks in advance for your thoughts.

[tobyink]

Type::Tie can be used to tie a scalar, array or hash variable to a type constraint. It supports Type::Tiny, Moose::Meta::TypeConstraint and MooseX::Types type constraints (and probably Mouse::Meta::TypeConstraint and MouseX::Types, though these are untested).

#!/usr/bin/env perl

use strict;
use warnings;

use Type::Utils qw(enum);
use Type::Tie;

my $scalar;
ttie $scalar, enum ["foo", "bar"];

$scalar = "foo";  # ok
$scalar = "bar";  # ok
$scalar = "baz";  # dies

Update: note though that tied scalars are significantly slower than untied. So an idea might be to tie your scalars in your development environment, and keep them untied in production. This can be achieved using something like:

use constant DEBUG => !!$ENV{DEVELOPMENT_SERVER};

my $scalar;
ttie $scalar, enum ["foo", "bar"] if DEBUG;

[jakeease]

The closest thing that I can think of is Data::Rlist. What you are asking sounds a bit like an enum. This discussion might be useful: How can I create an enum type in Perl?

[Anonymous Monk]

:) some food for thought :) Type::Tiny::Enum,Type::Tiny::Enum,Moose::Meta::TypeConstraint::Enum, http://search.cpan.org/dist/MooX-Types-MooseLike-0.23/lib/MooX/Types/MooseLike/Base.pm#Enum, Object::Enum, Class::Value::Enum

[Anonymous Monk]

Thanks jakeease and Anonymous for the feedback so far! I just thought I'd clarify: Yes, it's a bit like an enum - the thing is though that I'd like to retrofit some older scripts that already use a scalar as an enum. It'd be an advantage if I could just add a line at the top saying "restrict this scalar to this set of allowed values" to protect against future typos instead of updating every place where the scalar is used.

[sundialsvc4]

The only vague approximation that I have seen is use constant {};.   And it is a very poor substitute.   The Perl language simply does not have real enumerated-types or compile-time checking.