On 26/06/2009, at 9:36 PM, john skaller wrote:
> I don't like the STL iterator stuff much for Felix. So we have this
> little
> problem:
Just some notes: example: we can do this (which Haskell cannot):
////
typeclass A[T] {
virtual fun f: T -> T;
}
typeclass B[T] {
inherit A[T];
}
instance[T] A[T] { fun f: T -> T = "$1"; }
instance A[int] { fun f: int -> int = "$1+1"; }
open[T] B[T];
println$ f 1;
println$ f 1.3;
////
Note that instances (specialisations) can be fully polymorphic,
and also that overlapping instances are allowed provided one
is more specialised (fully contained in) the other. This this
code prints
2
1.3
A polymorphic (non-)specialisation like the first instance given
is similar to a virtual with a default body. But you cannot instantiate
f in B, even though it is inherited. In particular, you can't do this:
typeclass B[T] {
fun g...
fun f(x)=>g(x);
}
which is what we want: actually we want A to be Container
and declare virtual iterate, then define fold_left using it.
In B= Sequence we define iterate using STL begin/end
functions.
We see here that whilst a Sequence "is-a" Container,
derivation (inheritance) isn't the same as specialisation.
C++ (template) classes can solve this (for member functions).
There's a difference between specialising a class (inheritance)
and specialising its type variables (instantiation).
--
john skaller
[email protected]
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