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Package "maybe"

Title:The maybe type represents an optional value
Rating:Not rated. Create the first rating!
Latest version:0.1.1
SHA1 sum:11bfc624970bd914d2a4f2bf2f851e4653e28db9
Author:Michael Hendricks <michael@ndrix.org>
Maintainer:Michael Hendricks <michael@ndrix.org>
Packager:Michael Hendricks <michael@ndrix.org>
Home page:http://packs.ndrix.com/maybe/index.html
Download URL:http://packs.ndrix.com/maybe/maybe-0.1.1.tgz


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:- use_module(library(maybe)).
% staff(Name, Spouse)
staff(tom, just(teresa)).
staff(bob, nothing).
staff(sue, just(william)).

?- staff(Name, MaybeSpouse),
   maybe_default_value(MaybeSpouse, '(none)', Spouse).
 Name = tom,
 Spouse = teresa ;
 Name = bob,
 Spouse = '(none)' ;
 Name = sue,
 Spouse = william.


The maybe type encapsulates an optional value. When a value is present, we have just(Value). When it's absent we have nothing. In some circumstances, this can be a more natural model than using Prolog failure. For example, one might model a nullable SQL column using maybe.

This module draws inspiration from similar libraries for Mercury and Haskell. We make predicates available even where it's clearer to use unification. That facilitates using maybe values with maplist and friends.

In addition to the predicates described below, this module defines clauses for the multifile predicate error:has_type/2 which describe the type maybe and maybe(T) where T is a type parameter. It also defines clauses for working with library(quickcheck).

Changes in this Version

  • Add meta_predicate declarations for
    • call_maybe/3
    • map_maybe/3
    • fold_maybe/4


Using SWI-Prolog 6.3 or later:

?- pack_install(maybe).

This module uses semantic versioning.

Source code available and pull requests accepted at http://github.com/mndrix/maybe

- Michael Hendricks <michael@ndrix.org>

Contents of pack "maybe"

Pack contains 7 files holding a total of 10.0K bytes.