[Haskell-cafe] ANN: HLint 1.2

[Jan-Willem Maessen]

On Jan 12, 2009, at 9:01 AM, Duncan Coutts wrote:

> No because the current definition are recursive and ghc cannot inline
> recursive functions.
>
> map :: (a -> b) -> [a] -> [b]
> map _ []     = []
> map f (x:xs) = f x : map f xs
>
> It has to be manually transformed into a version that is not recursive
> at the top level:
>
> map :: (a -> b) -> [a] -> [b]
> map f = go
>  where
>    go []     = []
>    go (x:xs) = f x : go xs
>
> Then the map can be inlined at the call site and the 'f' inlined into
> the body of 'go'.

This seems like exactly the sort of mechanical transformation that  
computers do quickly and accurately, and humans get wrong.  Surely it  
wouldn't be that hard for GHC to transform self recursion in this way  
(possibly subject to the condition that the result be worth inlining)?

[phc did this, and I think it was inherited from Lennart's program  
transformations.]

-Jan-Willem Maessen