[mercury-users] Determinism detection for different notations.

[Bartlomiej Szymczak]

Hi.

Consider the following excerpt from my code:

:-pred uselessWord(string::in) is semidet.

uselessWord("a").
uselessWord("her").

:-type yesno ---> yes ; no.

:-pred isu(string::in,yesno::out) is det.
isu(W,X):-(if uselessWord(W) then X=yes else X=no).

This code works and compiles OK. However, I find the syntax very ugly,
so at first I was trying more FOL-like notation for isu
predicate:

:-pred isu(string::in,yesno::out) is det.
isu(W,yes):-uselessWord(W).
isu(W,no):- \+uselessWord(W).

But with such a definition, I get a compiler error:

john.m:140: In `isu(in, out)':
john.m:140:   error: determinism declaration not satisfied.
john.m:140:   Declared `det', inferred `nondet'.
john.m:141:   call to `john.uselessWord(in)' can fail.
john.m:142:   Negated goal can succeed.
john.m:141:   Disjunction has multiple clauses with solutions.

Clearly something is wrong, as both programs work the same way. I
suspect that the compiler doesn't see the connection between
uselessWord(W) and \+uselessWord(W). It thinks that both could succeed
or both could fail. In fact when one succeeds, the other fails and
vice-versa.

I just find the latter notation nice and the first one ugly, as in
predicate logic I would write:

uselessWord(a)
uselessWord(her)
forall W(isu(W,yes) <- uselessWord(W))
forall W(isu(W,no) <- ~uselessWord(W))

This looks like the not-compiling example. The compiling one doesn't
look like logic, as there is no if-then-else in logic. Neither there
is infix = operation in logic.

Any chance you could add better determinism detection in mercury so
that I could use my preferred notation?

Best regards,
-- 
Bartlomiej Antoni Szymczak

Gmail: rhywek at gmail.com
Work mail: bas at imm.dtu.dk
Student mail: s041702 at student.dtu.dk
DTU office: 321/129
DTU tel.: +4545253892
Mobile: +4520789323
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