Found some info on Typeclassopedia (https://wiki.haskell.org/Typeclassopedia) - see below. It sounds like this is basically a "function with one parameter already applied". The parameter is then available for further usage.. ---- ((->) e) (which can be thought of as (e ->); see above), the type of functions which take a value of type e as a parameter, is a Functor. As a container, (e -> a) represents a (possibly infinite) set of values of a, indexed by values of e. Alternatively, and more usefully, ((->) e) can be thought of as a context in which a value of type e is available to be consulted in a read-only fashion. This is also why ((->) e) is sometimes referred to as the reader monad; more on this later. ...then: As mentioned earlier, ((->) e) is known as the reader monad, since it describes computations in which a value of type e is available as a read-only environment. The Control.Monad.Reader module provides the Reader e a type, which is just a convenient newtype wrapper around (e -> a), along with an appropriate Monad instance and some Reader-specific utility functions such as ask (retrieve the environment), asks (retrieve a function of the environment), and local (run a subcomputation under a different environment). Martin Martin Vlk:
Hi, this one is a bit of a mystery to me.
I am working on solutions to cis194/NICTA Haskell exercises and among others I am asked to work with (->).
I understand it refers to the reader/environment design patterns, but I am stumped as for what exactly it is. It doesn't seem to be an ordinary type, I know it is used as type constructor in type declarations, so that suggests it's a function, but somehow special. Looking in Hoogle it seems to be Haskell keyword - e.g. not an ordinary function.
So what is it - type constructor, a keyword, how is it related to the reader/environment pattern?
From GHCI's :info (->) I get: data (->) a b -- Defined in ‘ghc-prim-0.4.0.0:GHC.Prim’ instance P.Monad ((->) r) -- Defined in ‘GHC.Base’ instance P.Functor ((->) r) -- Defined in ‘GHC.Base’ instance P.Applicative ((->) a) -- Defined in ‘GHC.Base’ instance P.Monoid b => P.Monoid (a -> b) -- Defined in ‘GHC.Base’
Cheers Martin