[ ghc-Bugs-643878 ] sortBy not stable

[Volker Stolz]

In local.glasgow-haskell-bugs, you wrote:
> The library report requires that sortBy be stable.  In
> 5.04.1 it isn't:

[Simon, I must have missed that ghc-bugs is subscribers-only]
There's an #ifdef'ed version in there which is stable, but the
newer mergesort is not:

#ifdef USE_REPORT_PRELUDE
sort = sortBy compare
sortBy cmp = foldr (insertBy cmp) []
#else

sortBy cmp l = mergesort cmp l
sort l = mergesort compare l

{-
Quicksort replaced by mergesort, 14/5/2002.

nhc98 has a stable mergesort, I'm told. Maybe we should ask Malcolm
nicely...
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