On Thu, Jan 15, 2009 at 1:44 PM, Wouter Swierstra
Would you call function composition (on endofunctions) "appending"? The join of a monad? A semi-colon (as in sequencing two imperative statements)? How do you append two numbers? Addition, multiplication, or something else entirely?
All these operations are monoidal, i.e., are associative and have both left and right identities. If that's exactly what they have in common, why invent a new name? "Appendable" may carry some intuition, but it is not precise and sometimes quite misleading.
I think this highlights an interesting point: Haskell is more abstract than most other languages. While in other languages "Appendable" might just mean what Brian suggested in his post, "something with an empty state and the ability to append things to the end", in Haskell it applies to numbers and everything else that has an associative operator, that is, everything that is a monoid. So "Appendable" for numbers would be quite wrong; either it would never be used in situations where one wanted to "append things to the end" (which is limiting), or it would be used in these situations, which would be quite confusing. I think it is much more important to have good documentation about the typeclasses (and everything else in the library). This issue was mentioned recently in a discussion about monads, and the documentation for the Haskell library is quite uninformative. It would be nice if 1) people would not be scared of names like "monoid" and "functor" and 2) the documentation clearly stated what these things are, in a programming context, preferably with some examples. I think 2 would mitigate some of the fear mentioned in 1, if newcomers started to experience things like "hey, that's one funky-named stuff this monoid thing, but I see here in the documentation that it is quite simple". -- []s, Andrei Formiga