dons:
> 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.
>
Oh, I should note the inlining only happens here since the list constant
is a 'build', and map is a bulid . foldr, so we get a build/foldr
fusion, and an inlined map as a result.
If we just use map in isolation, no inlining:
A.foo =
\ (xs_ala :: [Char]) ->
map @ Char @ Char toUpper xs_ala
Whereas a worker/wrapper version
map :: (a -> b) -> [a] -> [b]
map f xs = go xs
where
go [] = []
go (x:xs) = f x : go xs
{-# INLINE map #-}
We get an inlined version:
go =
\ (ds_dm7 :: [Char]) ->
case ds_dm7 of wild_B1 {
[] -> [] @ Char;
: x_all xs_aln ->
:
@ Char (toUpper x_all) (A.go xs_aln)
}
-- Don
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