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] allbery@kf8nh.com

system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu

electrical and computer engineering, carnegie mellon university KF8NH