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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
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constraint-classes

This package reimplements various type classes for types that have constraints. Constraints are represented using ConstraintKinds and the classes are constructed in a way that is supposed to not clash with the regular ones.

Motivation

Many types constructors have fundamental constraints in their domains. For example, storable vectors can only contain elements that are Storable. In these cases, no class instances can be done and overloading is completely lost.

This package tries to solve this problem while:

  • being compatible with Prelude
  • being as seamless to use as possible
  • preserving type inference

These constraints make a more general, subhask-like solution not desirable.

Architecture

The domain of a type function f is represented by a type family Dom f a of kind Constraint. The domain can be cartesian or closed, represented by classes DomCartesian and DomClosed containing the appropriate constraint entailments.

The class functions use the domain:

_fmap :: (CFunctor f, Dom f a, Dom f b) => (a -> b) -> f a -> f b

and the domain classifiers:

_zipA :: (CApply f, DomCartesian f, Dom f a, Dom f b) => f a -> f b -> f (a, b)

when necessary.

The general rule of translation from standard classes is:

  • classes are prefixed with a C: Class becomes CClass
  • functions are prefixed with a _: function becomes _function
  • operators are suffixed with :: * becomes *:

Currently the implemented classes are, approximately from base, which some changes:

  • CFunctor
  • CApply
  • CApplicative
  • CMonad
  • CAlt
  • CAlternative
  • CFoldable
  • CTraversable

From keys:

  • CLookup
  • CIndexable
  • CKeyed
  • CZip
  • CZipWithKey
  • CFoldableWithKey
  • CTraversableWithKey
  • CAdjustable

Problems

An instance for Apply for a functor whose domain is not cartesian closed needs to have _ap = undefined. This is not a problem since any use of _ap will not compile due to the lack of DomClosed instance, but it's ugly.

Monads are supposed to be endofunctors, which is not the case here, I'm not sure how to interpret that.

TODO

Improve the architecture, better choice of classes, more classes, more instances.