# Type witness

### From HaskellWiki

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− | A '''type witness''' is a value that represents one of a range of possible | + | A '''type witness''' is a value that represents one of a range of possible [[type]]s. When implemented as [[generalised algebraic datatype]]s (GADTs), type witness can be used to perform dynamic casts. |

− | Types <tt>a</tt> and <tt>b</tt> may or may not be the same type. We wish to perform the cast < | + | Types <tt>a</tt> and <tt>b</tt> may or may not be the same [[type]]. We wish to perform the cast <hask>a -> Maybe b</hask>, or more generally, <hask>p a -> Maybe (p b)</hask> for any <tt>p</tt>, depending on whether <tt>a</tt> and <tt>b</tt> are the same type. This can be done with GADTs if we have "witnesses" for the two types: |

− | + | <haskell> | |

+ | dynamicCast :: Witness a -> Witness b -> p a -> Maybe (p b) | ||

+ | </haskell> | ||

A simple witness type might be defined over the Int, Bool and Char types like so: | A simple witness type might be defined over the Int, Bool and Char types like so: | ||

− | + | <haskell> | |

− | + | data Witness a where | |

− | + | IntWitness :: Witness Int | |

− | + | BoolWitness :: Witness Bool | |

+ | CharWitness :: Witness Char | ||

− | + | dynamicCast IntWitness IntWitness pa = Just pa | |

− | + | dynamicCast BoolWitness BoolWitness pa = Just pa | |

− | + | dynamicCast CharWitness CharWitness pa = Just pa | |

− | + | dynamicCast _ _ _ = Nothing | |

+ | </haskell> | ||

− | If we wish to add type | + | If we wish to add [[type constructor]]s such as the list constructor, that can be done by composing type constructors: |

− | + | <haskell> | |

− | + | data Witness a where | |

− | + | IntWitness :: Witness Int | |

− | + | BoolWitness :: Witness Bool | |

− | + | CharWitness :: Witness Char | |

+ | ListWitness :: Witness a -> Witness [a] | ||

− | + | data Compose p q a = MkCompose (p (q a)) | |

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− | + | ||

+ | dynamicCast IntWitness IntWitness pa = Just pa | ||

+ | dynamicCast BoolWitness BoolWitness pa = Just pa | ||

+ | dynamicCast CharWitness CharWitness pa = Just pa | ||

+ | dynamicCast (ListWitness wa) (ListWitness wb) pla = do | ||

+ | MkCompose plb <- dynamicCast wa wb (MkCompose pla) | ||

+ | return plb | ||

+ | dynamicCast _ _ _ = Nothing | ||

+ | </haskell> | ||

+ | By using more complex composition types in addition to <tt>Compose</tt>, it is possible for witnesses to allow type constructors of more complex [[kind]]s. | ||

[[Category:Idioms]] | [[Category:Idioms]] |

## Latest revision as of 20:03, 16 March 2006

A **type witness** is a value that represents one of a range of possible types. When implemented as generalised algebraic datatypes (GADTs), type witness can be used to perform dynamic casts.

`a`and

`b`may or may not be the same type. We wish to perform the cast

a -> Maybe b

p a -> Maybe (p b)

`p`, depending on whether

`a`and

`b`are the same type. This can be done with GADTs if we have "witnesses" for the two types:

dynamicCast :: Witness a -> Witness b -> p a -> Maybe (p b)

A simple witness type might be defined over the Int, Bool and Char types like so:

data Witness a where IntWitness :: Witness Int BoolWitness :: Witness Bool CharWitness :: Witness Char dynamicCast IntWitness IntWitness pa = Just pa dynamicCast BoolWitness BoolWitness pa = Just pa dynamicCast CharWitness CharWitness pa = Just pa dynamicCast _ _ _ = Nothing

If we wish to add type constructors such as the list constructor, that can be done by composing type constructors:

data Witness a where IntWitness :: Witness Int BoolWitness :: Witness Bool CharWitness :: Witness Char ListWitness :: Witness a -> Witness [a] data Compose p q a = MkCompose (p (q a)) dynamicCast IntWitness IntWitness pa = Just pa dynamicCast BoolWitness BoolWitness pa = Just pa dynamicCast CharWitness CharWitness pa = Just pa dynamicCast (ListWitness wa) (ListWitness wb) pla = do MkCompose plb <- dynamicCast wa wb (MkCompose pla) return plb dynamicCast _ _ _ = Nothing

By using more complex composition types in addition to `Compose`, it is possible for witnesses to allow type constructors of more complex kinds.