On 20/02/2012, at 12:11 PM, john skaller wrote:
> ////////////////////////////
> sthe_name : = "@" sthe_name =># "(Prefix)";
small white lie: you would need
syntax fred { sthe_name : = "@" sthe_name =># "(Prefix)"; }
open syntax fred;
in user code. Now more magic:
typedef ptrs = typesetof ( &int, @int );
body """
void pr (void *p) {
printf("%p\\n",p);
}
""";
proc pr: !ptrs = "pr($1);";
pr x;
This procedure will accept either &int or @int argument.
This saves doing a conversion.
Unfortunately, typesets don't work quite right with polymorphism,
I couldn't get this to work for arbitrary T. Now we have a use case
I can look at how the compiler handles polymorphic constraints.
However consider we also want this:
fun id[T] : @T -> @T = "$1";
fun id[T] : &T -> &T = "$1";
This is a pain. We either have to use a typeclass, which is function specific,
or we have to define multiple overloads. This is because Felix has no subtyping.
In this case is we knew &T was a sort of subtype of @T we could say that in the
second case the return type and argument were covariant, and that should be
enough for the type to be restricted by the compiler.
BTW: another typeset based extension to the type system
intrigues me:
fun div: int * (int - {0}) -> int; // division
fun div(x:int, y:int):int where x!=0 => ...
There's some interesting theory here: the function isn't total in the form
fun div: int * int -> int
because of division by zero. But we can either subtract out the illegal 0 from
the domain as shown, or, make it a pre-condition .. which is exactly the same
thing OR we can ADD a value to the return type:
fun div: int * int -> int + error;
fun div: int * int -> opt[int] // same idea as above
This makes this div total, and the restriction to the int - {0} case is
also total. The transformation here is a standard category theory square.
match denom with
| 0 -> None
| _ -> Some (x/y)
endmatch
this is related to deref of null pointers (I think the pair of function is an
adjoint).
--
john skaller
skal...@users.sourceforge.net
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