[m-users.] [Beginner] How to make a function using list pattern matching deterministic

[Volker Wysk]

Hi again

What you seem to have misunderstood: It's all *possible* calls which make
the determinism category, not all *actual* calls. 

Volker

Am Freitag, dem 14.10.2022 um 13:31 +0200 schrieb Volker Wysk:
> Am Freitag, dem 14.10.2022 um 16:29 +0530 schrieb Razetime:
> > Currently I have these two predicates in my program that I think I can
> > make deterministic.
> > 
> > :- pred entry(list(string)::in, list(int)::out) is det.
> > entry(Dirs, Out) :-
> >   map((pred(X::in, [L,R]::out) is det :-
> >          split(X, 1, L, R)),
> >        Dirs, Spl),
> >   foldl2((pred([T, B]::in, D::in, Ds::out, P::in, Ps::out) is det :-
> >             rot(T, D, Ds),
> >             BI = string.det_to_int(B),
> >             map((pred(X::in, Y::out) is det :- Y = X*BI), Ds, Dso),
> >             map_corresponding((pred(X::in, Y::in, Z::out) is det :- Z
> > = X+Y), P, Dso, Ps)
> >         ), Spl, [0, 1], _, [0, 0], Out).
> > 
> > :- pred rot(string::in, list(int)::in, list(int)::out) is det.
> > rot("L", [X, Y], [A, B]) :-
> >   A = -Y,
> >   B =  X.
> > rot("R", [X, Y], [A, B]) :-
> >   A =  Y,
> >   B = -X.
> 
> You're defining rot/3 to take two element lists, as the second argument,
> only. It can't be deterministic, because of what should happen when no such
> argument was specified, but, say an empty list. The first argument also
> isn't exhaustive.
> 
> You could use unexpected/2 from the "require" library:
> 
> 
> :- pred rot1(string::in, list(int)::in, list(int)::out) is semidet.
> 
> rot1("L", [X, Y], [A, B]) :-
>   A = -Y,
>   B =  X.
> rot1("R", [X, Y], [A, B]) :-
>   A =  Y,
>   B = -X.
> 
> 
> :- pred rot(string::in, list(int)::in, list(int)::out) is det.
> 
> rot(LR, Inlist, Outlist) :-
>    ( if   rot1(LR, Inlist, Outlist0)
>      then Outlist = Outlist0
>      else unexpected($pred, "Unexpected arguments in call to rot/3")
>    ).
> 
> 
> It could be made to accept only two-element list and make it deterministic,
> with the help of insts. But that's advanced stuff.
> 
> 
> Bye,
> Volker
> 
> > 
> > I'm having trouble with both of these since they're giving me problems
> > with determinism inference:
> > 
> > a01.m:017: In `entry'(in, out):
> > a01.m:017:   Determinism error in lambda expression.
> > a01.m:017:   Declared `det', inferred `semidet''.
> > a01.m:017:   In argument 1 of clause head:
> > a01.m:017:   in argument 2 of functor `[|]/2':
> > a01.m:017:   in argument 2 of functor `[|]/2':
> > a01.m:017:   unification with `list.[]' can fail.
> > a01.m:017:   In argument 1 of clause head:
> > a01.m:017:   in argument 2 of functor `[|]/2':
> > a01.m:017:   unification with `list.[B | V_35]' can fail.
> > a01.m:017:   In argument 1 of clause head:
> > a01.m:017:   unification of `LambdaHeadVar__1' and `list.[T | V_34]' can fail.
> > a01.m:024: In `rot'(in, in, out):
> > a01.m:024:   error: determinism declaration not satisfied.
> > a01.m:024:   Declared `det', inferred `semidet'.
> > a01.m:024:   The reasons for the difference are the following.
> > a01.m:025:   The switch on HeadVar__1 can fail.
> > a01.m:025:   In argument 2 of clause head:
> > a01.m:025:   in argument 2 of functor `[|]/2':
> > a01.m:025:   in argument 2 of functor `[|]/2':
> > a01.m:025:   unification with `list.[]' can fail.
> > a01.m:025:   In argument 2 of clause head:
> > a01.m:025:   in argument 2 of functor `[|]/2':
> > a01.m:025:   unification with `list.[V_23 | V_22]' can fail.
> > a01.m:025:   In argument 2 of clause head:
> > a01.m:025:   unification of `HeadVar__2' and `list.[V_21 | V_20]' can fail.
> > 
> > how do i make sure that these functions that use a pattern match are
> > det? Adding a branch with ; for the error cases makes it multi, but I
> > want to know how i can make it return a single value for every set of
> > arguments.
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> 
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