From: "Iavor Diatchki"
I know this is not exactly accurate, but for beginners' purposes one could explain 'Functor' as a container type such as a list or a tree.
Only to students who already know what a (parameterised) list and tree ARE. Introducing both those concepts, and generalising them first over arbitrary element types, and then over the kind of container, is something that will take some time. It's on the way to that point that one needs to be able to talk about and use lists, without encountering Functor even in error messages.
In any case, if one wanted to avoid the issue, it seems entirely reasonable to define a function 'mapList' that has the list specific type. Most classes probably do this anyway.
That would be the alternative, but I doubt whether any classes do that right now. What's the point, when map already does the job?
From: Jon Fairbairn
A reasonable choice (forestalling the objection that using List.map, listMap or mapList would be too distracting for students) would be
lmap:: (t -> t') -> [t] -> [t'] map:: Functor f => (t -> t') -> f t -> f t'
I'd prefer mapList to lmap, to be honest--at least the connection between the name and the type is clear. But remember, this would at a stroke make Haskell' incompatible with all existing Haskell textbooks. Even if publishers brought out new editions, even if we told students to buy them, there are many, many second hand books in circulation, and it would be years before one could rely on students having Haskell' books. All that time, students would write map instead of mapList because that's what the book says, and get stuck with incomprehensible error messages. Is it really worth an incompatible change in the library functions used by all beginners, just to rename fmap to map? It seems to me that the gain from a change is very small, and the cost considerable. John