[Haskell-cafe] whine and solution about programmers not respecting documentations

Daniel Fischer daniel.is.fischer at web.de
Tue Jun 29 04:57:53 EDT 2010

On Tuesday 29 June 2010 10:47:40, Zura_ wrote:
> Maybe it is because deleteBy is defined wrongly? i.e. it is not logical,
> doesn't follow the common sense user might expect. It accepts any
> predicate but narrows requirements only in docs.

Unfortunately, you can't easily encode the requirement that it ought to be 
an equivalence relation via types. So putting it in the docs and hoping 
nobody gets burned too badly is probably the best one can do.

> Maybe best could be to just take a value for comparison and use "=="
> against it?

That's the plain delete.

> ("overloaded" or "built-in" (I'm a C++ engineer but as I
> know it is possible in Haskell as well.))
> And to have separate delete which accepts arbitrary predicate. (As it is
> now.)
> Regards,
> Zura
> Albert Y. C. Lai wrote:
> > The doc of deleteBy states: "The deleteBy function behaves like
> > delete, but
> > takes a user-supplied equality predicate." A precondition is that the
> > user-supplied predicate is an equality predicate. (>=) is not an
> > equality predicate, be it in the layperson sense of "it isn't
> > analogous to (==)" or the
> > mathematical sense of "it isn't an equivalence relation".

More information about the Haskell-Cafe mailing list