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

On 29.06.10 08:37, Ketil Malde wrote:

Albert Y.C.Lai 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.