Original-Via: uk.ac.nsf; Tue, 26 Nov 91 02:54:29 GMT
If I have understood you correctly, there is not a problem.
If there is an instance declaration for class C and type T, it must occur
either in the module that declares C or the module that declares T, and is
exported whenever C or T is exported. So whenever C and T are in scope, so
is the C-T instance, and your problem cannot arise. (report 5.3.1)
--brian
Suppose I define
f x y z = if y==z then x*x else x
the typing will be
f :: Num a, Eq b => a -> b -> b -> a
with a and b quantified. As far as I understand, the instantiations of Eq
Num in scope at this point are irrelevant in the meaning of f.
The rationale (presumably) is that since f is a closure, we might as
well wait until it is applied to circumscribe its applicability.
Now suppose I define
g = f 3::Int
the type for g will be
Num Int, Eq b => b -> b -> Int
My understanding is that, if Int is not an instance of Num at this
point, an error will be declared (although g is also a function, and
there is no particular reason not to wait until it is applied before
checking that Int is an instance of Num). This of course ignores
the fact that Int is declared an instance of Num in the prelude.
I guess I have two questions:
1. Am I right in my interpretation of the meaning of contexts?
Is there some place in the report where this is explained?
2. If I am, what is the rationale for this decision? It seems to me
that if such checking is delayed at all, it ought to be delayed until
it is time to print something!
Satish
