Re: [Vala] Trap for the unwary or incorrect code?

[Al Thomas <astavale yahoo co uk>]

----- Original Message -----
> From: Andy Lees <andrewl oz gmail com>
> Sent: Saturday, 21 May 2016, 14:56
> Subject: [Vala] Trap for the unwary or incorrect code?
>
> If I have the following code, or similar:
> void main (string[] args) {
>   uint ui = 2;
>   int i = -3;
>   if (i < ui) {
>     stdout.printf("Comparison works\n");
>   }
> }
>
> No output is produced.  Given Vala is generally very good at warning about
> type mismatches, or correctly doing an implicit cast, I find this rather
> strange.  Yes, I should not be comparing a negative value against an
> unsigned value, but logically the comparison should produce the expected

> output, I would have thought?
> > In general, as type inference can make the type of a variable unclear to
> the casual viewer, and the declaration of a variable may be in a different
> file, this permissiveness is a potential source of error, I think.



At present I would say this is a trap for the unwary, although a 

--strict-value-type mode for Vala may be helpful. This is a well known problem
in the C world.

In Vala the spec for relational arithmetic operators says "Where both operands 

are of integer type, both are converted to the largest integer type involved"
https://wiki.gnome.org/Projects/Vala/Manual/Expressions#Relational_operations
Presumably that means the largest value rather than range of values.

You will also find this with arithmetic operations, e.g. 

http://stackoverflow.com/questions/35305625/vala-quotient-of-two-integers-is-always-an-integer-why

In C this type of behaviour is labelled the "usual arithmetic conversions" if
you want to do a search. Some examples showing what you have found:
https://www.securecoding.cert.org/confluence/display/c/INT02-C.+Understand+integer+conversion+rules
http://www.iesensor.com/blog/2012/10/21/unsigned-integer-can-be-evil-c-implicit-conversion/

Al