Portability | portable |
---|---|

Stability | experimental |

Maintainer | Edward Kmett <ekmett@gmail.com> |

Safe Haskell | Safe-Inferred |

# Tagged values

A

value is a value `Tagged`

s b`b`

with an attached phantom type `s`

.
This can be used in place of the more traditional but less safe idiom of
passing in an undefined value with the type, because unlike an `(s -> b)`

,
a

can't try to use the argument `Tagged`

s b`s`

as a real value.

Moreover, you don't have to rely on the compiler to inline away the extra argument, because the newtype is "free"

Typeable2 Tagged | |

Monad (Tagged s) | |

Functor (Tagged s) | |

Functor (Tagged s) => Applicative (Tagged s) | |

Foldable (Tagged s) | |

(Functor (Tagged s), Foldable (Tagged s)) => Traversable (Tagged s) | |

Bounded b => Bounded (Tagged s b) | |

Enum a => Enum (Tagged s a) | |

Eq b => Eq (Tagged s b) | |

(Fractional (Tagged s a), Floating a) => Floating (Tagged s a) | |

(Num (Tagged s a), Fractional a) => Fractional (Tagged s a) | |

(Real (Tagged s a), Enum (Tagged s a), Integral a) => Integral (Tagged s a) | |

(Typeable (Tagged s b), Data s, Data b) => Data (Tagged s b) | |

Num a => Num (Tagged s a) | |

(Eq (Tagged s b), Ord b) => Ord (Tagged s b) | |

Read b => Read (Tagged s b) | |

(Num (Tagged s a), Ord (Tagged s a), Real a) => Real (Tagged s a) | |

(RealFrac (Tagged s a), Floating (Tagged s a), RealFloat a) => RealFloat (Tagged s a) | |

(Real (Tagged s a), Fractional (Tagged s a), RealFrac a) => RealFrac (Tagged s a) | |

Show b => Show (Tagged s b) | |

(Ord (Tagged s b), Ix b) => Ix (Tagged s b) | |

Monoid a => Monoid (Tagged s a) |

retag :: Tagged s b -> Tagged t bSource

Some times you need to change the tag you have lying around.
Idiomatic usage is to make a new combinator for the relationship between the
tags that you want to enforce, and define that combinator using `retag`

.

data Succ n retagSucc :: Tagged n a -> Tagged (Succ n) a retagSucc = retag

asTaggedTypeOf :: s -> Tagged s b -> sSource

`asTaggedTypeOf`

is a type-restricted version of `const`

. It is usually used as an infix operator, and its typing forces its first argument (which is usually overloaded) to have the same type as the tag of the second.