[Haskell-cafe] ANN: HLint 1.2

[Don Stewart]

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