Re: "where" block local to a guard?

17 Sep 2002


      At 3:07 PM +0930 9/17/02, Dr Mark H Phillips wrote:
...
Suppose you have some function
functn :: Int -> Int
functn i
    | i>5       = t  * i
    | i>0       = t_ * i
    | otherwise = 1
    where
    t  = functn (i-2)
    t_ = functn (i-1)
Notice that t and t_ are really local to a guard, rather
than to the whole guard section.  Why then, can't you write:
functn :: Int -> Int
functn i
    | i>5       = t * i
        where
        t = functn (i-2)
    | i>0       = t * i
        where
  t = functn (i-1)
    | otherwise = 1
In particular, the above would mean you wouldn't need two names
t and t_, you could just use t for both!
Am I doing something wrongly, or is there a good reason why
where isn't allowed to be used in this way?
You can get the effect you're after by using let-expressions:
...
functn :: Int -> Int
 functn i
     | i>5       = let t = functn (i-2) in t * i
     | i>0       = let t = functn (i-1) in t * i
     | otherwise = 1
'where' is part of the syntax of definitions, not expressions. This 
enables a name defined in a where-clause to be used in more than one 
guarded expression.

--Ham
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Hamilton Richards                Department of Computer Sciences
Senior Lecturer                  The University of Texas at Austin
512-471-9525                     1 University Station C0500
Taylor Hall 5.138                Austin, Texas 78712-1188
ham@cs.utexas.edu                hrichrds@swbell.net
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