[Haskell-cafe] Re: [Haskell-beginners] pattern matching on date type

[Thomas Davie]

On 1 Jan 2009, at 09:36, Max.cs wrote:

> 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'
Hi again Max,
   I'm assuming this is continuing from the concatT example, and that  
you're struggling with first function you must pass to foldTree.   
Remember the type of the function – it's not a -> Tree a, but Tree a - 
 > Tree a, because your leaves in the parent tree all contain trees to  
glue on at that point.

So, the function you want, is the function which looks at the  
parameter it's given, goes "oh, that's interesting", does nothing to  
it, and hands it back to replace the Leaf.   I recommend searching  
hoogle for functions of type a -> a, the function you're looking for  
is built in.

Bob