Re: [Pnet-developers] Problems in ternary with upcasting

[Rhys Weatherley]

On Friday 04 July 2003 06:31 pm, Gopal V wrote:

> Any suggestions anyone ?

Currently, "ValidateStack" checks to see if A can be coerced to B, or B can be 
coerced to A for a type-O validation (verify_branch.c).  This is how it is 
defined in the C# language, but it looks like the runtime engine needs to be 
a little more flexible.

The verifier probably needs to be changed to search up the object hierarchy 
for a common parent type.  It shouldn't be too hard to do.  Something like 
this:

TryCommonType(A, B) ->
    if A can be coerced to B, return B
    if B does not have a parent, then return "Object"
    otherwise TryCommonType(A, parent(B))

CommonType(A, B) ->
    A1 = TryCommonType(A, B)
    A2 = TryCommonType(B, A)
    if A1 can be coerced to A2, then return A1
    otherwise return A2

Note: the last step of CommonType returns the lowest type in the hierarchy 
that is common.  Otherwise it will always return "Object" (if we assume that 
"Object" is top-most in the hierarchy).

It may also be necessary to check for interface inheritance.  I'm not too sure 
on that front.  Some experimentation will be required probably.

Cheers,

Rhys.