Portability | generalized newtype deriving |
---|---|
Stability | experimental |
Maintainer | Edward Kmett <ekmett@gmail.com> |
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
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.