[mercury-users] Puzzled about about I/O in predicates which may fail

[Julien Fischer]

On Wed, 23 Nov 2004, Gregory D. Weber wrote:

> It seems to me that by using state variables, I can "fix" an
> error involving I/O in predicates which may fail.
> If read_problem_decl_semidet is indeed semidet, then
>
> (EXAMPLE 1)
>
> main(!IO) :-
> 	(
> 	  read_problem_decl_semidet("credit.decl", Problem, !IO) ->
> 	  describe_problem(Problem, !IO)
> 	;
> 	  io__print("Error reading file\n", !IO)
> 	).
>
> compiles without any error message.  Moreover, running the program
> actually works!
>
This is a bug in the compiler.  Using the IO state in the condition of an
if-then-else should always be an error but the current compiler only
detects it sometimes.

Cheers,
Julien.
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