Haskell's syntax

[Johannes Waldmann]

> (3) Types and expressions look very much alike. Say we have:
>   data Tree a = Bin (Tree a) a (Tree a) | Nil.

I encourage my students to always use named record notation

  data Tree a = Bin { left :: Tree a, ... } 


> Around half (!) of the students now write (on a written test):
>   size (Bin (Tree left) x (Tree right)) = ...

sure they had this in their mind:

  size (Bin ( left :: Tree a ) ( x :: a ) ( right :: Tree a )) = ...

(i. e. they wanted to specify the types of formal parameters
at the most natural place - at their first appearance in the text)


I think the following is conceptually cleaner

  size ( t @ Bin { } ) = 
       let l = left t
	   r = right t
       in  size l + 1 + size r

(it separates matching and binding)
altough the first line looks rather cryptic.


NB: I wouldn't mind the language enforcing the students ( and me ) to write 

  size ( t @ Bin { } :: Tree a ) = 
       let l :: Tree a = left t
	   r :: Tree a = right t
       in  size l + 1 + size r

Best regards,
-- 
-- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ --
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