[Haskell-cafe] ANN: HLint 1.2

[Don Stewart]

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