[project @ 2001-07-20 10:05:37 by simonpj]

--------------------------
	Correct a bug in exprArity
	--------------------------

This long-standing bug meant that exprArity gave over-pessimistic
answers.  e.g.

	\x -> f x

where nothing is known about f.  We were getting

	0 (for f)
	-1 (for the argument)
	+1 (for the lambda)
	-----
	0

The right answer is of course 1.

diff --git a/ghc/compiler/coreSyn/CoreUtils.lhs b/ghc/compiler/coreSyn/CoreUtils.lhs
index bea8316..d483b82 100644 (file)
--- a/ghc/compiler/coreSyn/CoreUtils.lhs
+++ b/ghc/compiler/coreSyn/CoreUtils.lhs
@@ -790,15 +790,17 @@ And in any case it seems more robust to have exprArity be a bit more intelligent
 
 \begin{code}
 exprArity :: CoreExpr -> Int
-exprArity e = go e `max` 0
+exprArity e = go e
            where
              go (Lam x e) | isId x        = go e + 1
                           | otherwise     = go e
              go (Note _ e)                = go e
              go (App e (Type t))          = go e
-             go (App f a) | exprIsCheap a = go f - 1
+             go (App f a) | exprIsCheap a = (go f - 1) `max` 0
                -- Important!  f (fac x) does not have arity 2, 
                --             even if f does!
+               -- NB: `max 0`!  (\x y -> f x) has arity 2, even if f is
+               --               unknown, hence arity 0
              go (Var v)                   = idArity v
              go _                         = 0
 \end{code}