Hi Martin, You certainly know what (->) is, but you might be confused about its prefix notation. (->) is a type constructor which represent a normal Haskell function. That is,(->) A B represents a type of a function from type A to type B. Since (->) is a right-associative infix operator we almost always write it in the infix form: a -> b -> c, which means a -> (b -> c), which means in the prefix form:(->) a ((->) b c). Similarly + is the normal, infix addition operator, but written in parenthesis you can write it in prefix form, that is: (+) 2 2 == 1 + 3. When you import Control.Monad.Reader, ghci will tell you that: Prelude Control.Monad.Reader> :info (->) data (->) a b -- Defined in `GHC.Prim' instance Monad ((->) r) -- Defined in `GHC.Base' instance Functor ((->) r) -- Defined in `GHC.Base' instance MonadReader r ((->) r) That is a type contructor (still not a complete type!) (->) r is of class MonadReader r. In other words, a function which accepts type r may act as a reader from environment of type r. It might be easier to understand after reading its implementation. instance MonadReader r ((->) r) where ask = id local f m = m . f Best, Grzegorz Milka On 23.07.2015 10:07, Martin Vlk wrote:
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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners