2009/3/19 Claus Reinke
If the map, filter, fold, etc can be unrolled, then the unrolled definitions would be fused, right? So what is missing is fine control ("how much to unroll this particular call to map here").
The issues is that In stream fusion the combinators like "map" are all non-recursive and so unrolling/peeling doesn't make any sense. In fact, their being non-recursive is almost the whole point, because it lets GHC inline them like crazy and hence build a nice efficient fused pipeline of combinators eventually. The recursion is introduced purely in one place - unstream - and even then it doesn't go through unstream but through a locally recursive wrapper (so GHC can see the structure of the stream). So, it might be sufficient if: 1) You changed stream fusion so unstream was directly recursive, but added an INLINE PEEL 1 annotation to it, so if the call site doesn't do any unrollling at least you will still be able to spot the structure of the stream 2) You could, at the call site, add an INLINE PEEL 1 UNROLL n annotation that took the /original/ RHS for unstream and unrolled it however many times the user specifies (you still need a PEEL 1 so you can spot the stream structure in the unrolled loop) Unfortunately, this all feels quite fragile :-( Max