[m-users.] comprehension question

[Volker Wysk]

Hi!

I have a question. I don't understand, why this doesn't work. This is
the backwards direction of error() from the posix library:

:- pred error1(int, posix.error).
:- mode error1(out, in) is det.
%:- mode error1(in, out) is det.

error1(Num, Res) :-   ((  Num = 0,    Err = e2BIG
    ;   Num = 1,    Err = eACCES
    ;   Num = 2,    Err = eAGAIN
    ;   Num = 3,    Err = eBADF
(...)
    ;   Num = 39,   Err = eSPIPE
    ;   Num = 40,   Err = eSRCH
    ;   Num = 41,   Err = eTIMEDOUT
    ;   Num = 42,   Err = eXDEV
    ) ->
        Res = Err
    ;
        Res = unknown(Num, "unknown errno")
    ).

I get this message from the compiler:

strerror.m:072: In clause for `error1(out, in)':
strerror.m:072:   scope error: attempt to bind a non-local variable
inside the
strerror.m:072:   condition of an if-then-else.
strerror.m:072:   Variable `Num' has instantiatedness `free',
strerror.m:072:   expected instantiatedness was `unique(0)'.
strerror.m:072:   The condition of an if-then-else is only allowed to
bind
strerror.m:072:   variables which are local to the condition or which
occur
strerror.m:072:   only in the condition and the `then' part.


Shouldn't this work in both directions? I think it's logical...?

Bye
Volker