From 98a74c9da8b10a3ba8fa2c261c2e26970c56a811 Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Fri, 20 Jul 2001 10:05:37 +0000
Subject: [PATCH] [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.
---
 ghc/compiler/coreSyn/CoreUtils.lhs |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/ghc/compiler/coreSyn/CoreUtils.lhs b/ghc/compiler/coreSyn/CoreUtils.lhs
index bea8316..d483b82 100644
--- 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}
-- 
1.7.10.4