# Multi-parameter type class

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− | The correct LANGUAGE pragma for them is <hask>{-# LANGUAGE MultiParamTypeClasses #-}</hask> |
+ | To enable them, use the <hask>{-# LANGUAGE MultiParamTypeClasses #-}</hask> pragma. |

If you think of a single-parameter type class as a set of types, then a multi-parameter type class is a relation between types. |
If you think of a single-parameter type class as a set of types, then a multi-parameter type class is a relation between types. |

## Revision as of 17:19, 21 December 2012

## 1 About

Basically, type classes which can take multiple arguments, such as:

class Monad m => VarMonad m v where new :: a -> m (v a) get :: v a -> m a put :: v a -> a -> m () instance VarMonad IO IORef where ... instance VarMonad (ST s) (STRef s) where ...

{-# LANGUAGE MultiParamTypeClasses #-}

If you think of a single-parameter type class as a set of types, then a multi-parameter type class is a relation between types.

Naive use of MPTCs may result in ambiguity, so functional dependencies were developed as a method of resolving that ambiguity, declaring that some subset of the parameters is sufficient to determine the values of the others.

Some uses of MPTCs with functional dependencies can be replaced with type families.