thanks!
suppose we have
> data Tree a = Leaf a | Branch (Tree a) (Tree a)
deriving Show
and how I could define a function foo :: a -> Tree a that
foo a = Leaf
a where a is not a type of
Tree
foo b =
b where
b is one of the type of Tree (Leaf or Branch) ?
The following code seems not working......
foo (Leaf a) = a
foo a = Leaf a
saying 'Couldn't match expected type `a' against
inferred type `Btree a''
any idea?
Thanks,
Max
Sent: Thursday, January 01, 2009 7:35 AM
Subject: Re: [Haskell-cafe] definition of data
On 2009 Jan 1, at 2:32, Max.cs wrote:
data Tree a = a | Branch (Tree a) (Tree a)
deriving Show
but it seems not accpetable in haskell ?
You need a constructor in both legs of the type:
> data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
--
electrical and computer engineering, carnegie mellon
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