diff options
Diffstat (limited to 'compiler/coreSyn/CoreUtils.lhs')
| -rw-r--r-- | compiler/coreSyn/CoreUtils.lhs | 24 |
1 files changed, 22 insertions, 2 deletions
diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs index 7017f709cb..9b527e7fcf 100644 --- a/compiler/coreSyn/CoreUtils.lhs +++ b/compiler/coreSyn/CoreUtils.lhs @@ -1712,8 +1712,14 @@ tryEtaReduce bndrs body --------------- fun_arity fun -- See Note [Arity care] - | isLocalId fun && isStrongLoopBreaker (idOccInfo fun) = 0 - | otherwise = idArity fun + | isLocalId fun + , isStrongLoopBreaker (idOccInfo fun) = 0 + | arity > 0 = arity + | isEvaldUnfolding (idUnfolding fun) = 1 + -- See Note [Eta reduction of an eval'd function] + | otherwise = 0 + where + arity = idArity fun --------------- ok_lam v = isTyVar v || isEvVar v @@ -1737,6 +1743,20 @@ tryEtaReduce bndrs body ok_arg _ _ _ = Nothing \end{code} +Note [Eta reduction of an eval'd function] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In Haskell is is not true that f = \x. f x +because f might be bottom, and 'seq' can distinguish them. + +But it *is* true that f = f `seq` \x. f x +and we'd like to simplify the latter to the former. This amounts +to the rule that + * when there is just *one* value argument, + * f is not bottom +we can eta-reduce \x. f x ===> f + +This turned up in Trac #7542. + %************************************************************************ %* * |