[Haskell-cafe] Announce: ghc-core, command line pager for reading GHC Core

[Duncan Coutts]

On Fri, 2008-05-23 at 21:24 +0200, Henning Thielemann wrote:
> On Fri, 23 May 2008, Bulat Ziganshin wrote:
> 
> > Hello Henning,
> >
> > Friday, May 23, 2008, 8:31:24 PM, you wrote:
> >
> >> would guarantee speed in every case. Or I can SPECIALISE the function,
> >> then the function will only be called, but with polymorphism overhead
> >> eliminated. This would only work for a restricted range of types. I'd like
> >> to have a pragma, that tells GHC to specialise a function for every type
> >> it is called with.
> >
> > me too. btw, this already present in jhc. inlining doesn't work in any
> > complex case since recursive functions can't be inlined
> 
> GHC inlines recursive functions, too, otherwise it could not turn 'foldl' 
> and friends into plain machine loops.

Actually ghc's definition of foldl is not recursive, though it does of
course contain a local recursion:

foldl        :: (a -> b -> a) -> a -> [b] -> a
foldl f z xs = lgo z xs
             where
                lgo z []     =  z
                lgo z (x:xs) = lgo (f z x) xs

The lgo recursive call is then specialised at the call site and we can
get good code. 

As I understand it, if foldl was written in the standard directly way
then ghc would not inline it. So we have to manually apply the static
argument transformation. You'll see that foldr is written in the same
way.

Duncan