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Monad-Result zef:rawleyfowler last updated on 2023-02-25

bffc8c655d02744a673da1a8ca6d70587ff01a0d/

Monad-Result

An implementation of the result monad from OCaml. This allows the developer to completely avoid exceptions. Note the result type is not generic, and cannot be statically typed. I think this makes it more flexible overall, compared to OCaml's result monad. (Gradual typing ftw)!

How to use (default)

use Monad::Result;

sub my-error-causing-function($value --> Result) {
  return Monad::Result.error('Bad value!') if $value eq 'bad'
  ok('Value was good!');
}

# Bind operator: >>=:
my-error-causing-function('bad') >>=: -> $value {
  say $value; # Is not called because this is not bindable (it is an error!)
  return Monad::Result.ok($value);
};

my-error-causing-function('abc') >>=: -> $value {
  say $value; # Is said because this is bindable, it is OK!
  return Monad::Result.ok($value);
};

# Map operator: >>=?
# Since this call results in OK, we can map it!
my $result = (my-error-causing-function('abc') >>=? -> $value {
  $value ~ ' HELLO WORLD';
}); # Results Ok('abc HELLO WORLD');

say $result.is-ok; # True
# NOTE UNWRAP IS DANGEROUS (In this case we know it's okay though!)
say $result.unwrap; # 'abc HELLO WORLD';

How to use (subs)

Subs exist as a separate import, because they interfere with a lot of common packages.

use Monad::Result :subs;

sub my-error-causing-function($value --> Result) {
  return error('Bad value!') if $value eq 'bad'
  ok('Value was good!');
}

# Bind operator: >>=:
my-error-causing-function('bad') >>=: -> $value {
  say $value; # Is not called because this is not bindable (it is an error!)
  return ok($value);
};

my-error-causing-function('abc') >>=: -> $value {
  say $value; # Is said because this is bindable, it is OK!
  return ok($value);
};

# Map operator: >>=?
# Since this call results in OK, we can map it!
my $result = (my-error-causing-function('abc') >>=? -> $value {
  $value ~ ' HELLO WORLD';
}); # Results Ok('abc HELLO WORLD');

say $result.is-ok; # True
# NOTE UNWRAP IS DANGEROUS!
say $result.unwrap; # 'abc HELLO WORLD';

How to install

zef install Monad-Result

LICENSE

Monad::Result is provided under the Artistic-2.0 license.