[mercury-users] determinism not satisfied

[Ralph Becket]

> From: Terrence Brannon [mailto:tmbranno at oracle.com]
> Sent: 11 July 2001 09:34
> 
> If anyone could help me get this simple program to work, I would
> appreciate it. Program and compilation error messages follow.
> 
> % p.14 "Clause and Effect" by William Clocksin
> 
> :- module border.
> :- interface.
> :- import_module io.
> 
> :- pred main(io__state::di, io__state::uo) is det.
> 
> :- implementation.
> :- import_module std_util.
> 
> main -->
> 	{ border("sussex",L) }, io__write_string(L).
> 
> border("sussex","kent").
> border("sussex","surrey").
> 
> adjacent(X,Y) :- border(X,Y).
> adjacent(X,Y) :- border(Y,X).
> 
> affordable(X,Y) :- adjacent(X,Z), adjacent(Z,Y).

You haven't supplied type or mode declarations for these predicates.

> [localhost:mercury/lexparse/border] metaperl% mmc --infer-all border.m
> border.m:015: Inferred :- pred border(string, string).
> border.m:018: Inferred :- pred adjacent(string, string).
> border.m:021: Inferred :- pred affordable(string, string).

So the compiler has inferred their types for you.

> border.m:007: In `main(di, uo)':
> border.m:007:   error: determinism declaration not satisfied.
> border.m:007:   Declared `det', inferred `nondet'.
> border.m:013:   call to `border(di, uo)' can fail.
> border.m:013:   call to `border(di, uo)' can succeed more than once.

And it looks like it's had a go at inferring modes, too, although
these look a bit strange to me.

> border.m:013: In clause for `main(di, uo)':
> border.m:013:   in argument 2 of call to predicate
`io:write_string/3':
> border.m:013:   mode error: variable `DCG_0' has instantiatedness
`mostly_unique',
> border.m:013:   expected instantiatedness was `unique'.

This is a nondeterministic program, but main/2 must be either det or
cc_nondet or cc_multi.

> border.m:015: Inferred :- mode border(di, uo) is nondet.

Ah yes, here we go.

Here's what you want:

:- module border.
:- interface.
:- import_module io.

:- pred main(io__state::di, io__state::uo) is cc_nondet.

:- implementation.
:- import_module string.

:- pred border(string, string).
:- mode border(in, out) is nondet.
:- mode border(out, in) is nondet.

border("sussex", "kent").
border("sussex", "surrey").
...

:- pred adjacent(string, string).
:- mode adjacent(in, out) is nondet.

adjacent(X, Y) :- border(X, Y).
adjacent(X, Y) :- border(Y, X).

:- pred affordable(string, string).
:- mode affordable(in, out) is nondet.

affordable(X, Y) :- adjacent(X, Z), adjacent(Z, Y).

- Ralph
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