Re: [Haskell-cafe] Why Maybe exists if there is Either?

On Thu, Jan 9, 2014 at 12:12 PM, Brandon Allbery wrote:

On Thu, Jan 9, 2014 at 11:52 AM, Vlatko Basic wrote:

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... what exactly is the problem with using appropriate data types in appropriate places ... It is a good thing. Very good. But I do not think all library writers are using it in correct places. But that's another problem.

In Haskell it's even a solved one: use an appropriate typeclass. We have several error-like typeclasses. Admittedly, if some library does not take advantage of this your recourse may be to submit a patch.

And you still seem to be fixated on the notion that Maybe is only for errors. What exactly is the error in "user did not provide an optional parameter"?

Indeed. I use Maybe for non-errors, namely optional values, all the time. To compare with some other languages: Maybe is like a nil vs. non-nil pointer. Either is like returning two values/a tuple of Foo and Error. (For that matter, I sometimes use Either when a value can be either something or the other, but neither of them are errors.)