[Haskell-cafe] Closed Classes

[Lyle Kopnicky]

Notwithstanding module Y, I don't think you should infer in module X 
that g::Int (or, rather, Int->Int).  Since f is defined in a type class, 
it should be polymorphic, and so should g.  When you apply g to a type, 
it will check to see what instances are available, and match only if Int 
is the type of the variable.  But there's no reason to restrict the type 
of g itself.

- Lyle

camarao at dcc.ufmg.br wrote:

>Informally, what I see as the defining rule for "closed world" is: "an
>expression is typed according to the set of definitions that are
>visible in the context in which it is used". Other possibilities
>exist, but the nice thing about this is that it is an extension of
>what happens without overloading.
>
>With this definition, given
>   _______________________    __________________________
>  | module X (f,g) where  |   | module Y where         |
>  |                       |   |                        |
>  | class A a where       |   | import X               |
>  |       f :: a -> a     |   |                        |
>  | instance A Int where  |   | instance A Float where |
>  |       f = id          |   |       f x = x + 1.0    |
>  |                       |   |                        |
>  | g x = f x             |   | h x = g x              |
>  -------------------------   --------------------------
>
>we would infer: g::Int (since the context in g's definition has only
>f:Int) and thus h::Int in Y (since the context in h's definition has
>only one g::Int). If "h" was defined as "h x = f x" in Y, *then* it
>would have a polymorphic type (because there are two instances of "f"
>in Y).
>
>Carlos
>
>
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