[Haskell-cafe] Capitalization and associated type families

[Jonathan Cast]

On Mon, 2008-08-04 at 19:51 +0100, Brian Hulley wrote:
> Hi,
> I'm familiar with the capitalization rules for identifiers in Haskell 
> and know that they are very useful and practical e.g. by allowing 
> variables to be distinguished from "constants" in patterns etc.
> 
> However I'm trying to understand on a much deeper level whether or not 
> they can really be justified and indeed if there is a danger that they 
> can lead to inconsistency in the light of future extensions to the language.
> 
> For example, why is there any distinction between a type "variable" and 
> a type "constant"?
> 
> To make this clearer consider the type of (map):
> 
> 	(a -> b) -> List a -> List b
> 
> (ignoring the special [] syntax for list types which would just confuse 
> the issue here).
> 
> With explicit quantifiers, we'd write:
> 
> 	forall a b. (a -> b) -> List a -> List b
> 
> Now this begs the question: why does "List" need to start with a 
> capital? (supposing that quantifiers were always written explicitly)

AFAIK, it doesn't; the variable/constant distinction is just there to
let the implementation put the foralls in for you.  Similarly, if we had
explicit foralls on equations, we could say

forall n x xn. drop (n+1) (x:xn) = drop n xn

but

forall n. drop n nil = nil

and it would be perfectly clear that `nil' in the latter case was a
constructor.

Of course, if you want to introduce this rule, you'd better start out
with it and be consistent --- require every variable to be explicitly
brought into scope.  I think you'd find it pretty annoying after trying
to program in your new language for a while, though.

jcc