Henning Thielemann wrote:
Anyway, I tried to wrap Prelude lists in a newtype and thus got GHC (still 6.4.1) to invoke my rules instead of the Prelude rules. But I encountered the following problem: I define something like
nonFusable x y = fusable (aux x y)
where fusion rules are defined for 'fusable', but not for 'nonFusable'. I hoped that 'nonFusable' will be inlined and then 'fusable' is fused with other expressions. This does not happen. If I state the function definition also as rule, then GHC fuses eagerly.
I suspect that fusable and/or aux are inlined into nonFusable when the latter is compiled. That's too early - you want nonFusable (with the simple definition above) to be inlined into the client code first. Adding {-# INLINE nonFusable #-} should take care of this.
Analogously I observed that usage of ($) and (.) blocks fusion, and when I add the rules
"unfold-dollar" forall f x. f $ x = f x ;
"unfold-dot" forall f g. f . g = \x -> f (g x) ;
then fusion takes place as expected.
That shouldn't be necessary, these two ought to be inlined. Do you have a concrete example where this happens? Roman