[M3devel] IsAssignable

[Tony Hosking]

Base type of an enumeration is itself!

On 9 Jan 2010, at 21:05, Jay K wrote:

> Current:
> 
> 
> PROCEDURE IsAssignable (a, b: T): BOOLEAN =
>   VAR i, e: T;  min_a, max_a, min_b, max_b, min, max: Target.Int;
>   BEGIN
>     IF IsEqual (a, b, NIL) OR IsSubtype (b, a) THEN
>       RETURN TRUE;
>     ELSIF IsOrdinal (a) THEN
>       (* ordinal types:  OK if there is a common supertype
>          and they have at least one member in common. *)
>       IF IsEqual (Base(a), Base(b), NIL)
>          AND GetBounds (a, min_a, max_a)
>          AND GetBounds (b, min_b, max_b) THEN
>         (* check for a non-empty intersection *)
>         min := min_a;  IF TInt.LT (min, min_b) THEN min := min_b; END;
>         max := max_a;  IF TInt.LT (max_b, max) THEN max := max_b; END;
>         RETURN TInt.LE (min, max);
>       ELSE
>         RETURN FALSE;
>       END;
> 
> my proposed:
> 
> 
> 
> PROCEDURE IsAssignable (a, b: T): BOOLEAN =
>   VAR i, e: T;  min_a, max_a, min_b, max_b, min, max: Target.Int;
>   BEGIN
>     IF IsEqual (a, b, NIL) OR IsSubtype (b, a) THEN
>       RETURN TRUE;
>     ELSIF IsOrdinal (a) THEN
>       (* ordinal types:  OK if they have at least one member in common. *)
>       IF GetBounds (a, min_a, max_a)
>          AND GetBounds (b, min_b, max_b) THEN
>         (* check for a non-empty intersection *)
>         min := min_a;  IF TInt.LT (min, min_b) THEN min := min_b; END;
>         max := max_a;  IF TInt.LT (max_b, max) THEN max := max_b; END;
>         RETURN TInt.LE (min, max);
>       ELSE
>         RETURN FALSE;
>       END;
> 
> 
> Your proposed:
> 
> > IF (IsEqual (Base(a), Base(b), NIL)
> > OR IsEqual (Base(a), Int.T, NIL) AND IsEqual (Base(b), LInt.T, NIL)
> > OR IsEqual (Base(a), LInt.T, NIL) AND IsEqual (Base(b), Int.T, NIL))
> > AND GetBounds (a, min_a, max_a)
> > AND GetBounds (b, min_b, max_b) THEN
> 
> What's the point of checking the types? Given that they are both ordinal?
> To restrict certain assignments?
> Aren't the base types of any ordinal type either Int.T or LInt.T?
>  So the check will always be true?
> I have to check.
> 
> 
>  - Jay
> 

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