Optimising words

Donald Bruce Stewart dons at cse.unsw.edu.au
Mon Jul 9 22:13:39 EDT 2007

> Hi
> Actually, you can go slightly better:
> words s     = case dropWhile isSpace s of
>                  [] -> []
>                  (s:ss) -> (s:w) : drop1 sss
>                      where
>                          (w, sss) = break isSpace ss
>                          drop1 [] = []
>                          drop1 (x:xs) = words xs
> Although a good optimising compiler may make this last leap all on its own.
> >To really convince yourself and everyone else you could compare it
> >against the spec, for both total and partial values.
> I'm pretty convinced that I applied reasoning rules at each stage. If
> you have a proof, testing is irrelevant :-)
> I've already modified the Yhc base library with this optimisation, and
> done some basic testing, and nothing broke.

I'd second Duncan here -- strictness properties are *really* hard, so
changes to existing (H98) functions must come with both QuickCheck and
strictness checking properties, before I'd be comfortable accepting them.

The missing support for partial values in QuickCheck is quite a hole
that needs fixing.

-- Don

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