CODEWITHSUNDEEP
haskell
DisplayName
DisplayName

Reputation: 249

What is the resolution to this "No Instance" error?

I have created new type describing Binary Tree

data BinTree a = Null | Num a (BinTree a) (BinTree a)
deriving (Show)

And have created the following function:

treehandle :: BinTree a -> Bool
treehandle a = True

to check at least inputing values.

When I input value Null, program outputs result successfully, but I can't input binary tree. I try thus: 

treehandle (5 (Null) (Null))

but obtain:

<interactive>:66:13:
No instance for (Num (BinTree a1 -> BinTree a2 -> BinTree a0))
  (maybe you haven't applied enough arguments to a function?)
  arising from the literal ‘5’
In the expression: 5
In the first argument of ‘treehandle’, namely ‘(5 (Null) (Null))’
In the expression: treehandle (5 (Null) (Null))

Why?

Upvotes: 1

Views: 137

Answers (3)

DisplayName
DisplayName

Reputation: 249

I thank for your replies. I appreciate it. Haskell is "wild" language being mix of Lisp and Prolog. Gradually I begin to get used to it. I would share with my results. So, that's BinTree declaration:

data BinTree a = Leaf | Val a (BinTree a) (BinTree a)
 deriving (Show)

It resembles on Prolog unification algorithm - I mean about definition of "reverse" as below:

reverse :: BinTree a -> BinTree a
reverse (Val a1 (a2) (a3)) = (Val a1 (reverse a3) (reverse a2))
reverse Leaf = Leaf

It works!

Upvotes: 0

steve
steve

Reputation: 281

I would find a different naming for the data constructors if I was you. Num is also the name of a type class, this can be very confusing when looking at error messages.

Also deriving Show is not correctly indented, and you forgot the data constructor in treehandle (5 (Null) (Null)). Here is a working version.

data BinTree a = Leaf | Node a (BinTree a) (BinTree a) deriving Show

treehandle :: BinTree a -> Bool
treehandle _ = True

test = treehandle $ Node 5 Leaf Leaf

treehandle wants a value of type BinTree a and all you gave it was an Int and two empty BinTree's, it actually tried to apply the Int with the two empty BinTree's and failed. You have to make a Node to get a single BinTree a which you can pass to treehandle

Upvotes: 6

Silvio Mayolo
Silvio Mayolo

Reputation: 70277

You forgot the value constructor's name

treehandle (Num 5 (Null) (Null))

Upvotes: 11

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