Generalize groupBy in a useful way?

Wolfgang Jeltsch g9ks157k at
Sat Sep 6 13:02:47 EDT 2008

Am Freitag, 5. September 2008 09:06 schrieb Bart Massey:
> […]

> I think it would be better to expand the definition of of
> groupBy such that the equality test is applied only to
> adjacent elements (with the arguments in proper order).
> This would not change the behavior for true equality
> predicates, but would permit cases like incp to have
> sensible behavior, which can occasionally be useful. (It was
> to me in this case: I was identifying straights in poker
> hands.)

I think it would be better forcing the function to be an equivalence relation 
using dependent types. ;-) 

In the absence of dependent types, I think that noone should ever call groupBy 
with a relation which is not an equivalence.  So for me, it really doesn’t 
matter what the function does if the precondition for calling it is not 

> […]

Best wishes,

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