Add PartialMonoid typeclass
#14
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It is sometimes useful to consider partial semigroups that also have an identity element, say `pmempty`. Many, but not all, partial semigroups that have instances in this library are also partial monoids, with some identity element. This patch introduces a typeclass, `PartialMonoid`, that represents such types. This typeclass is meant to directly mirror the standard `Monoid` typeclass, prepending `pm` to each function in the standard typeclass and changing their types to represent partiality. The only deviation here is removing `pmappend` from the typeclass, since this is necessarily an alias for `<>?` specialized to monoids; unlike with the standard `mappend`, by moving this out of the typeclass, we avoid any possibility of unlawful instances where `pmappend ≢ (<>?)`. This patch tags each addition with a `@since 0.7.0.0` Haddock annotation, on the assumption that these changes will be included in a new release of the library with that version.
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Hi @chris-martin, if you have a chance, could you review this PR? |
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It is sometimes useful to consider partial semigroups that also have an identity element, say
pmempty. Many, but not all, partial semigroups that have instances in this library are also partial monoids, with some identity element. This patch introduces a typeclass,PartialMonoid, that represents such types. This typeclass is meant to directly mirror the standardMonoidtypeclass, prependingpmto each function in the standard typeclass and changing their types to represent partiality. The only deviation here is removingpmappendfrom the typeclass, since this is necessarily an alias for<>?specialized to monoids; unlike with the standardmappend, by moving this out of the typeclass, we avoid any possibility of unlawful instances wherepmappend ≢ (<>?).This patch tags each addition with a
@since 0.7.0.0Haddock annotation, on the assumption that these changes will be included in a new release of the library with that version.Fixes #13.