On 04/08/2012, at 2:46 PM, john skaller wrote:
>
> On 04/08/2012, at 11:36 AM, john skaller wrote:
>
>> I am implementing new instantiation rules and have run into a
>> nasty bug.
>
>
> This is the case that caused the problems:
>
> class S_expr
> {
> union sexpr[T] = | Leaf of T | Tree of list[sexpr[T]];
>
> instance[T with Str[T]] Str[sexpr[T]] {
> fun str(x:sexpr[T])=>
> match x with
> | Leaf ?a => str a
> | Tree ?b => str b
> ;
> }
> }
>
Removing S_expr from the library I now have everything building and working
including rank! This now prints 5 like it should:
//////////////
class Trick[T] {
virtual fun rank : 1 -> int;
}
instance [A,B] Trick[A -> B] {
fun rank() => 1 + Trick[B]::rank ();
}
instance [K] Trick [K] {
fun rank() => 1;
}
typedef int5 = int -> int -> int -> int->int;
println$ Trick[int5]::rank();
//////////////////
This is similar to recursive reduction in C++ templates,
where typically one uses an int, subtracts one in the
general case when recursing, and provides a ground
case with the int = 0 to stop the recursion.
Felix doesn't allow ints as "type variables" like C++ does.
So using a type like int5 above is a trick to overcome this
until we can provide the proper mechanism.
It may seem we can do this with unitsum types, if we could
subtract 1 from them, I may look at that. However the "right"
way to do this requires introducing a proper kinding system
and allowing "type variables" to be values of any kind,
not just the kind TYPE.
This has other uses, as mentioned type matches are currently
used for predicates on type:
T in typesetof (int, double)
means
typematch T with
| int => 1
| double => 1
| _ => 0
endmatch
however the result is a type, either 0 or 1 which stand in for
FALSE or TRUE which are types, i.e. values of kind TYPE.
They should not be -- you'd not want to use the result as an actual
type anywhere! As with C not having a bool type, values of 0 and 1
standing in for a real bool type, so 0 and 1 as types stand in for
a real BOOL kind.
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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