Revamping the numeric classes

Bjorn Lisper
Thu, 8 Feb 2001 09:45:09 +0100 (MET)

>> >Also, what is the inferred type of, for example
>> >    f x y = x + length y
>> >? It can be
>> >    Int -> [a] -> Int
>> >    [Int] -> [a] -> [Int]
>> >and neither is more general than the other. And this is a simple
>> >function.
>> Int -> [a] -> Int, since this is the type it will get in the original type
>> system.

>So I can't apply f to lists, but I could if I inline its body. This
>means that I cannot arbitrarily refactor a piece of code by moving
>parts of it into separate definitions: subexpressions are given
>some extra meanings only if they are physically placed in certain
>contexts. This is bad.

This is a misunderstanding. the transformation of f l y , where l :: [Int]
for instance, should depend only on the type of f and not its definition.
It is the call to f, not f itself, that becomes transformed. No inlining
takes place.

>Ah, so what uses of f are correct depends on its definition, not type!
>Sorry, this is way to radical.

>Types exist to formalize possible ways a value can be used. HM allows
>to determine most general types variables in a let-block (or: of a
>module) before their uses, so separate compilation is possible. In
>your system typechecking of a function's definition is done each time
>it is used!

No. See above.

Björn Lisper