[Haskell-cafe] pattern matching on date type

[Max.cs]

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



From: Brandon S. Allbery KF8NH 
Sent: Thursday, January 01, 2009 7:35 AM
To: Max.cs 
Cc: Brandon S. Allbery KF8NH ; beginn...@haskell.org ; haskell-cafe@haskell.org 
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


-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH


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[Haskell-cafe] definition of data

[Max.cs]

hi all, I want to define a data type Tree a that can either be  a  or Branch 
(Tree a) (Tree a)?

I tried  

data Tree a = a | Branch (Tree a) (Tree a) deriving Show

but it seems not accpetable in haskell ?

any way I could achieve this ?

Thanks

max___
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