[mercury-users] Prolog DCGs Pattern Matching in Mercury

Goncalo Jorge Coelho e Silva l10454 at alunos.uevora.pt
Mon Apr 28 08:49:05 AEST 2003

Hi again,

 I'd like to thank the replies to my previous
questions and bad coding. ;)

 I've been trying to explore how to use DCGs
in Mercury, for the same purpose as they are
used in Prolog's pattern matching. 

 Prolog example:

gce(name(N)) --> {nl, write(' matched "name(_something_)" '), nl},

gce(X) --> {X=[name(N)]},
        {nl, write(' matched "name(_something_)" '), nl},

 For that purpose, I made a quick test in

:- module inter.
:- interface.
:- import_module io.

:- pred main(io__state, io__state).
:- mode main(di, uo) is det.
%:- mode main(di, uo) is semidet.

:- pred seee(string, io__state, io__state).
:- mode seee(in, di, uo)is semidet.

:- implementation.
:- import_module string.

%main -->
        seee("faraway", IO0, IO1),
        %io__write_string("It's harder to see it.").
        io__write_string("It's harder to see it.", IO1, IO).

seee("faraway") --> io__write_string("Never ending!").
%seee(faraway) --> io__write_string("Never ending!").
%seee(VAR) --> {VAR="faraway"}, io__write_string("Never ending!").

 In this test example, I was trying to catch 
an occurence of the string "faraway" and 
print the sentence "Never ending!".

 (I've taken the suggestion of leaving DGC's notation in favour
of explicit state_of_the_world argument passing...)

 Which led me to the following errors:


inter.m:006: In `main(di, uo)':
inter.m:006:   error: determinism declaration not satisfied.
inter.m:006:   Declared `det', inferred `semidet'.
inter.m:020:   call to `seee(in, di, uo)' can fail.
make: *** [run] Error 1

 If I was to comment the 'det' mode, in favour
of the semidet:

%:- mode main(di, uo) is det.
:- mode main(di, uo) is semidet.
%:- mode main(di, uo) is cc_multi.

 I'd get:

inter.m:007: Error: main/2 must be `det' or `cc_multi'.
make: *** [run] Error 1

 What am I doing wrong, what can I do to
use Prolog DGCs lookalike effect in Mercury?

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