CODEWITHSUNDEEP
haskelltypescompiler-errorsfunctional-programming
BlaCkOmN
BlaCkOmN

Reputation: 81

Error expected type with actual type in Haskell

I declared these types in Haskell : 

data Tree x = Node x (Tree x) (Tree x)
          |Leef
          deriving Show

data Color = R | N
    deriving (Show, Eq)

data TreeRN x = Tree (Color,x)
    deriving Show

and this function : 

equilibre :: TreeRN a -> TreeRN a
equilibre (Node (N,z) (Node (R,y) (Node (R,x) a b) c) d) = (Node (R,y) (Node (N,x) a b) (Node (N,z) c d))
equilibre (Node (N,z) (Node (R,x) a (Node (R,y) b c)) d) = (Node (R,y) (Node (N,x) a b) (Node (N,z) c d))
equilibre (Node (N,x) a (Node (R,z) (Node (R,y) b c) d)) = (Node (R,y) (Node (N,x) a b) (Node (N,z) c d))
equilibre (Node (N,x) a (Node (R,y) b (Node (R,z) c d))) = (Node (R,y) (Node (N,x) a b) (Node (N,z) c d))

And here is my issue : 

Couldn't match expected type ‘TreeRN a’
            with actual type ‘Tree (Color, t3)’
Relevant bindings include
  d :: Tree (Color, t3) (bound at test.hs:15:53)
  c :: Tree (Color, t3) (bound at test.hs:15:51)
  z :: t3 (bound at test.hs:15:48)
  b :: Tree (Color, t3) (bound at test.hs:15:37)
  y :: t3 (bound at test.hs:15:34)
  a :: Tree (Color, t3) (bound at test.hs:15:23)
  equilibre :: TreeRN a -> TreeRN a (bound at test.hs:12:1)
  (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds)
In the expression:
  (Node (R, y) (Node (N, x) a b) (Node (N, z) c d))
In an equation for ‘equilibre’:
    equilibre (Node (N, x) a (Node (R, y) b (Node (R, z) c d)))
      = (Node (R, y) (Node (N, x) a b) (Node (N, z) c d))
Failed, modules loaded: none.

Usually when I get this kind of error I understand why and I can correct my function, but here I am disappoint by the 

Couldn't match expected type ‘TreeRN a’
            with actual type ‘Tree (Color, t3)’

while my declared type TreeRN is the same (for me), so I need answer for why the compiler say 't3', what does it mean ? It is really different to Tree (Color,a) ?

Upvotes: 1

Views: 124

Answers (1)

luqui
luqui

Reputation: 60463

It looks like you are expecting TreeRN x to be a synonym for Tree (Color, x). I think you have just used the wrong keyword -- data defines a new algebraic data type. Try

type TreeRN x = Tree (Color, x)

You will also need to remove the deriving clause which is not valid for type synonyms.

Upvotes: 3

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