[Haskell-cafe] Re: (flawed?) benchmark : sort

ajb at spamcop.net ajb at spamcop.net
Wed Mar 12 19:48:04 EDT 2008

G'day all.

Adrian Hey wrote:

>> This might be a reasonable thing to say about *sortBy*, but not sort
>> as the ordering of equal elements should not be observable (for any
>> correct instance of Ord). It should be impossible to implement a
>> function which can discriminate between [a,a],[a,b],[b,a],[b,b] if
>> compare a b = EQ.

Nonsense.  Consider a Schwartzian transform wrapper:

data OrdWrap k v = OrdWrap k v

instance (Ord k) => Ord (OrdWrap k v) where
     compare (OrdWrap k1 v1) (OrdWrap k2 v2) = OrdWrap k1 k2

It would be incorrect (and not sane) for sort [a,b] to return [a,a] in
this case, though a case could be made that either [a,b] or [b,a] make

Quoting Jules Bean <jules at jellybean.co.uk>:

> Stability is a nice property. I don't understand why you are arguing
> against this so aggressiviely.

Stability is an occasionally very useful property.  However, if there
is a tradeoff between stability and performance, I'd prefer it if the
library didn't choose for me.

Andrew Bromage

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