ndmitchell:
> Hi
>
> > Does GHC specialize map? If it doesn't, then hand crafted version
> > could be faster.
>
> GHC doesn't specialize map, and a hand-crafted one could be faster -
> but you then wouldn't get foldr/build fusion. In general HLint tries
> to make the code prettier, but sometimes you will need to deviate from
> its suggestions when you've profiled etc. To stop HLint warning you
> just create Hints.hs and include the line "ignore =
> LennartsSuperFastModule.mySpecialisedMap" - full details in the
> manual.
>
> >> I found so many 'map' re-implementations in Haskell libraries, even in
> >> those, where I thought their programmers must be more experienced than me.
> >> Hm, maybe even in libraries by Neil?
>
> I can't really be blamed for making mistakes before HLint ;-)
>
But GHC tends to inline and specialise map, due to:
"map" [~1] forall f xs.
map f xs = build (\c n -> foldr (mapFB c f) n xs)
So that,
main = print (map toUpper "haskell")
Yields:
s :: Addr#
s = "haskell"#
letrec
unpack_snX :: Int# -> [Char]
unpack_snX = \ (x :: Int#) ->
case indexCharOffAddr# s x of i {
_ -> ($wtoUpper i) (: @ Char) (unpack_snX (+# x 1)
'\NUL' -> [] @ Char
Which looks inlined and specialised to my eyes.
-- Don
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