Safe Haskell | Safe |
---|

# Documentation

A GADT witnessing equality of two types. Its only inhabitant is `Refl`

.

Typeable2 := | |

GRead (:= a) | |

GShow (:= a) | |

GCompare (:= a) | |

GEq (:= a) | |

Ord a => OrdTag (:= a) | |

Eq a => EqTag (:= a) | |

Read a => ReadTag (:= a) | In order to make a
The instance GRead Tag where greadsPrec _p str = case tag of "AString" -> [(\k -> k AString, rest)] "AnInt" -> [(\k -> k AnInt, rest)] _ -> [] where (tag, rest) = break isSpace str instance ReadTag Tag where readTaggedPrec AString = readsPrec readTaggedPrec AnInt = readsPrec |

Show a => ShowTag (:= a) | |

Eq (:= a b) | |

Ord (:= a b) | |

Read (:= a a) | |

Show (:= a b) |

A class for type-contexts which contain enough information to (at least in some cases) decide the equality of types occurring within them.

geq :: f a -> f b -> Maybe (a := b)Source

Produce a witness of type-equality, if one exists.

A handy idiom for using this would be to pattern-bind in the Maybe monad, eg.:

extract :: GEq tag => tag a -> DSum tag -> Maybe a extract t1 (t2 :=> x) = do Refl <- geq t1 t2 return x

Or in a list comprehension:

extractMany :: GEq tag => tag a -> [DSum tag] -> [a] extractMany t1 things = [ x | (t2 :=> x) <- things, Refl <- maybeToList (geq t1 t2)]

(Making use of the `DSum`

type from Data.Dependent.Sum in both examples)

defaultEq :: GEq f => f a -> f b -> BoolSource

If `f`

has a `GEq`

instance, this function makes a suitable default
implementation of '(==)'.

defaultNeq :: GEq f => f a -> f b -> BoolSource

If `f`

has a `GEq`

instance, this function makes a suitable default
implementation of '(/=)'.

data GOrdering a b whereSource

A type for the result of comparing GADT constructors; the type parameters of the GADT values being compared are included so that in the case where they are equal their parameter types can be unified.

weakenOrdering :: GOrdering a b -> OrderingSource

TODO: Think of a better name

This operation forgets the phantom types of a `GOrdering`

value.

class GEq f => GCompare f whereSource

Type class for orderable GADT-like structures. When 2 things are equal,
must return a witness that their parameter types are equal as well (GEQ).
|Type class for comparable GADT-like structures. When 2 things are equal,
must return a witness that their parameter types are equal as well (`GEQ`

).