[M3devel] Semantics of HasWideChars

[Olaf Wagner]

Quoting "Rodney M. Bates" <rodney.bates at wichita.edu>:

> CM3's Text.HasWideChars, as implemented, has some strange behavior.
> From Text.i3:
>
> PROCEDURE HasWideChars (t: T): BOOLEAN;
> (* Returns "TRUE" if "t" contains any "WIDECHAR" characters. *)
>
> This reads to me like it refers to the actual characters comprising t,
> which also makes sense to a client of this interface.
>
> In fact, the implementation returns TRUE if the internal representation
> used is capable of representing any wide characters, even though they
> could all be narrow.
>
> Some examples:
>
> Text.HasWideChars(W"ABC") = TRUE
>
> Text.HasWideChars(W"ABC"&"def") = TRUE
>
> Text.HasWideChars(Text.Sub(W"ABC"&"def",3,3)) = FALSE
>
> Text.HasWideChars(Text.Sub(W"ABC"&"def",2,3)) = TRUE
>
> These results have nothing to do with the abstract view of TEXT as
> a string of characters.  Moreover, a principal reason a client of
> Text would call HasWideChars would be to know whether it could
> subsequently call, e.g., Text.GetChar without having the value
> silently truncated to 8 bits.
>
> So I propose to fix HasWideChars.  This will entail some performance
> penalties, as in many cases, it will have to go through the character
> values one at at time in a loop and range-check each one.
>
> The alternative would be to redefine HasWideChars as a one-sided
> approximation, (which is really is now) i.e., such that a result
> of FALSE means the string  is guaranteed to contain no wide characters,
> but TRUE only means the real answer is unknown.  Not very useful, and in
> that case, it really should be be renamed MightHaveWideChars.
>
> What do others think?  Anybody using GetWideChars who would be
> affected?

I think it should be fixed. If anybody really needs the current semantics,
we can add the `might have' predicate later, but I don't see that it is
very useful ;-)

Olaf
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