On 29.06.10 08:37, Ketil Malde wrote:
Albert Y.C.Lai<tre...@vex.net> writes:
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".
One could argue that this is a bad specification. The type is
deleteBy :: (a -> a -> Bool) -> a -> [a] -> [a]
but there are further limitations on the arguments, and worse, the function
doesn't check this and produce an error if the arguments don't conform,
but just silently produces a meaningless result.
How can 'deleteBy' check that an argument is an equivalence relation?
(Putting aside that this harms performance.)
--
Best regards,
Roman Beslik.
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