[Haskell-beginners] 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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