Portability | non-portable |
---|---|
Stability | experimental |
Maintainer | Edward Kmett <ekmett@gmail.com> |
Safe Haskell | Trustworthy |
Reifies arbitrary terms at the type level. Based on the Functional Pearl: Implicit Configurations paper by Oleg Kiselyov and Chung-chieh Shan.
http://www.cs.rutgers.edu/~ccshan/prepose/prepose.pdf
The approach from the paper was modified to work with Data.Proxy and to cheat by using knowledge of GHC's internal representations by Edward Kmett and Elliott Hird.
Usage comes down to two combinators, reify
and reflect
.
>>>
reify 6 (\p -> reflect p + reflect p)
12
The argument passed along by reify is just a data
, so all of the information needed to reconstruct your value
has been moved to the type level. This enables it to be used when
constructing instances (see Proxy
t =
Proxyexamples/Monoid.hs
).
In addition, a simpler API is offered for working with singleton values such as a system configuration, etc.
Reflection
reify :: forall a r. a -> (forall s. Reifies s a => Proxy s -> r) -> rSource
Reify a value at the type level, to be recovered with reflect
.