| Portability | portable |
|---|---|
| Stability | experimental |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Safe Haskell | Safe-Infered |
Data.Tagged
Contents
Description
Tagged values
A value is a value Tagged s bb 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 bs 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"
Instances
| Typeable2 Tagged | |
| Monad (Tagged s) | |
| Functor (Tagged s) | |
| Applicative (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) | |
| Floating a => Floating (Tagged s a) | |
| Fractional a => Fractional (Tagged s a) | |
| Integral a => Integral (Tagged s a) | |
| (Data s, Data b) => Data (Tagged s b) | |
| Num a => Num (Tagged s a) | |
| Ord b => Ord (Tagged s b) | |
| Read b => Read (Tagged s b) | |
| Real a => Real (Tagged s a) | |
| RealFloat a => RealFloat (Tagged s a) | |
| RealFrac a => RealFrac (Tagged s a) | |
| Show b => Show (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.