[mercury-users] Learning Mercury

[Tom Davies]

Hi,

I'm learning Mercury by going through Bratko's Prolog programming for  
AI, and I have questions about his Monkey example.

The Prolog version is:
move(
	state(middle, onbox, middle, no),
	grasp,
	state(middle, onbox, middle, yes)
	).

move(
	state(P, onfloor, P, H),
	climb,
	state(P, onbox, P, H)
	).

move(
	state(P1, onfloor, P1, H),
	push(P1,P2),
	state(P2, onfloor, P2, H)
	).
	
move(
	state(P1, onfloor, B, H),
	walk(P1, P2),
	state(P2, onfloor, B, H)
	).

canget(state(_,_,_,yes)).

canget(S1) :-
	move(S1,Move,S2),
	canget(S2).
	
canget(state(door, onfloor, window, no)).

I have tried to convert this to Mercury (my Mercury code is at the  
end of this email), but get the error:
Blonder:~/dev/other/bratko tomd$ mmc -v
Mercury Compiler, version rotd-2007-07-27, configured for i686-apple- 
darwin8.10.1
...
Blonder:~/dev/other/bratko tomd$ mmc --infer-all -E --make monkey
Making Mercury/cs/monkey.c
monkey.m:052: In clause for predicate `monkey.canget'/1:
monkey.m:052:   warning: variable `Move' occurs only once in this scope.
monkey.m:037: In clause for `move(in, out, out)':
monkey.m:037:   mode error in conjunction. The next 3 error messages  
indicate
monkey.m:037:   possible causes of this error.
monkey.m:034:   In clause for `move(in, out, out)':
monkey.m:034:   in argument 2 of clause head:
monkey.m:034:   mode error in unification of `HeadVar__2' and  
`monkey.push(P1,
monkey.m:034:   P2)'.
monkey.m:034:   Variable `HeadVar__2' has instantiatedness `free',
monkey.m:034:   term `monkey.push(P1, P2)' has instantiatedness  
`monkey.push(
monkey.m:034:     ground,
monkey.m:034:     free
monkey.m:034:   )'.
monkey.m:034:   In clause for `move(in, out, out)':
... error log truncated, see `monkey.err' for the complete log.
** Error making `Mercury/cs/monkey.c'.

Am I giving the wrong modes for move?

Thanks for any advice.

Tom

:- module monkey.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module bool.
:- type horizontal_t ---> door; middle; window.
:- type vertical_t ---> onbox; onfloor.
:- type state_t --->
state(
	monkeyHorizontal::horizontal_t,
	monkeyVertical::vertical_t,
	boxHorizontal::horizontal_t,
	hasBanana::bool
	).
:- type action_t ---> grasp; climb; push 
(bfrom::horizontal_t,bto::horizontal_t);
					walk(mfrom::horizontal_t,mto::horizontal_t).

:- pred move(state_t,action_t,state_t).
:- mode move(in,out,out) is nondet.

move(
	state(middle, onbox, middle, no),
	grasp,
	state(middle, onbox, middle, yes)
	).

move(
	state(P, onfloor, P, H),
	climb,
	state(P, onbox, P, H)
	).

move(
	state(P1, onfloor, P1, H),
	push(P1,P2),
	state(P2, onfloor, P2, H)
	).
	
move(
	state(P1, onfloor, B, H),
	walk(P1, P2),
	state(P2, onfloor, B, H)
	).
	
:- pred canget(state_t).
:- mode canget(in) is nondet.

canget(state(_,_,_,yes)).

canget(S1) :-
	move(S1,Move,S2),
	canget(S2).
	
main(!IO) :-
	print(canget(state(door, onfloor, window, no)), !IO).
	



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