Wrong function declaration giving error when inferring on type
I have started learning Haskell from Learn You a Haskell. In one of the early parts there is an example with binary trees and I started thinking about implementing a remove function and hence got sidetrack开发者_运维技巧ed to zippers and am now looking at Zippers
As part of the exercises on the linked wiki page on zippers, I have functions with the following method declarations
get :: Node a -> a
put :: a -> Node a -> Node a
retrieveNode :: Thread -> Node a -> Node a
retrieve :: Thread -> Node a -> a
Now I try to implement the following function
update :: Thread -> Node a -> (a -> a) -> Node a
update t n f = put (f (get (retrieveNode t n)) retrieveNode t n) -- Line 29 referenced
Loading this in ghci gives:
Prelude> :l theseus.hs
[1 of 1] Compiling Main ( theseus.hs, interpreted )
theseus.hs:29:15:
Couldn't match expected type `Node a'
against inferred type `Node a1 -> Node a1'
In the expression:
put (f (get (retrieveNode t n)) retrieveNode t n)
In the definition of `update':
update t n f = put (f (get (retrieveNode t n)) retrieveNode t n)
Failed, modules loaded: none.
I read up on the Monomorphism restriction, but couldn't decide if this is relevant for my code.
You're missing the second argument to put
. From a glance it looks like you just got your parenthesis wrong. Try this.
update :: Thread -> Node a -> (a -> a) -> Node a
update t n f = put (f (get (retrieveNode t n))) (retrieveNode t n)
精彩评论