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haskelloperatorstypeclass

Haskell - custom operator for multi-parameter type class?

Suppose I have a multi-parameter type class

class MyClass a b where
    ...

foo :: MyClass a b => a -> b
foo = ...

Is it possible to define a custom infix symbol for this relation, so that I can instead write

infixr 1 ?
class a ? b where
    ...

foo :: a ? b => a -> b
foo = ...
Solution

  • Yes, with TypeOperators you can give a symbolic name to a class directly:

    {-# Language
    MultiParamTypeClasses,
    TypeOperators #-}

module Example
  ( type (?)
  …
  ) where

class a ? b where
  …

infixr 1 ?

foo :: a ? b => a -> b
foo = …

    Or you can make an infix alias for a named class using ConstraintKinds:

    {-# Language
    ConstraintKinds,
    MultiParamTypeClasses,
    TypeOperators #-}

module Example
  ( type (?)
  …
  ) where

class C a b where
  …

type a ? b = C a b

infixr 1 ?

    Note that you may need ExplicitNamespaces for importing this as well, if it’s ambiguous.

    Another alternative is an empty subclass, if you expect to extend it with additional constraints, since this tends to be better for inference:

    class (C a b) => a ? b
instance (C a b) => a ? b

infixr 1 ?