Turning (:) into (++) feels like the wrong way round to me. What's wrong with rewriting x : build g into build (\cn. x `c` g c n) as you suggest? Now, [x1,x2,x3] ++ ys is just syntactic sugar for (x1 : x2 : x2 : []) ++ ys and I suppose that optimising that into x1 : x2 : x3 : ys is a good thing quite independent of the (x : build g) stuff isn't it? Simon | -----Original Message----- | From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of David | Feuer | Sent: 31 July 2014 19:38 | To: Haskell Libraries | Subject: Seeking comments on proposed fusion rules | | The rules: | | lpaste.net/108508 | | The boring explanation: | | As I mentioned earlier, we currently can't fuse things like | | foldr (x : build g) | | The foldr can't see the build. Earlier, I considered a simple | cons/build rule that would rewrite | | x : build g | | to | | build (\c n -> x `c` g c n) | | I wasn't quite satisfied with this approach, largely because it treats | | foldr c n (x1 : x2 : ... : xn : build g) | | one way, and | | foldr c n ([x1, x2, ..., xn) : build g) | | another way. The former expands out the foldr, while the latter (via | various rules involving augment) translates into a foldr over [x1, x2, | ..., xn]. | | I therefore came up with the idea of translating x1:x2:...:xn:build g | into [x1:x2:..:xn]++build g, and then letting the current rules do | their thing. dolio and carter (in #ghc) were concerned about what | would happen if fusion *didn't* occur, in which case the resulting | code appears to be *worse*. So then I wrote some rules to fix that up, | which actually look likely to be good rules in general. They turn | | [x1:x2:...:xn] ++ ys | | into | | x1:x2:...:xn:ys | | I ended up having to do more that I had originally expected in my | rules, because they miss a phase they need (it may be possible to fix | that; I'm not sure, because I'm very new to this business); I left | comments showing each step of the translation. One thing of note is | that I ended up using (by hand) the augment/augment rule that is | commented out as probably not being useful. | | In my limited testing, the rules seem to do what they're supposed to | (when I can understand the Core), and the profiler is very pleased. | _______________________________________________ | Libraries mailing list | Libraries@haskell.org | http://www.haskell.org/mailman/listinfo/libraries