Implementation of a map function

Question

I'm trying to implement a Scheme-like map function, i.e.

map([X1, X2, ..], Fun) ->[Fun(X1), Fun(X2), ...]

I wrote this code:

map([], Fun, []).
map([H|T], Fun, [HO|TO]) :- call(Fun, H, HO), map(T,F,TO).

Now, looking at this run:

?- map([1,2,3], plus(1), X).
X = [2, 3, 4] ;
X = [2, 3, 4] ;
X = [2, 3, 4] ;
X = [2, 3, 4] ;
ERROR: map/3: Arguments are not sufficiently instantiated
   Exception: (9) map([3], _G380, _G351) ?

How can I make it stop after the first solution provided?
How can I trace it?If i use trace it stop the execution after the first solution.

Note that SWI-Prolog has a maplist predicate that does exactly what you want. — Fred Foo
– Fred Foo, Commented Aug 29, 2012 at 20:08

thanos · Accepted Answer · 2012-08-29 18:08:25Z

6

1) the code you posted has a problem: map(T,F,T0) should be map(T,Fun,T0). without this change i get the error you mentioned immediately; with this correction it runs flawlessly. (it is also a good idea to change map([], Fun, []) to map([],_Fun,[]) since you dont use the variable Fun - you should get a warning for singleton variables)

2) when you trace it and reach the first solution, press ;. alternatively, in swi-prolog, press the spacebar instead of enter and the tracing will continue.

6 ?- trace.
true.

[trace] 6 ?- X = 1 ; X = 2.
   Call: (7) _G522=1 ? creep
   Exit: (7) 1=1 ? creep
X = 1 ;
   Call: (7) _G522=2 ? creep
   Exit: (7) 2=2 ? creep
X = 2.

answered Aug 29, 2012 at 18:08

thanos

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3 Comments

Aslan986 Over a year ago

Thank you for your help. I changed F n Fun. It does not give me that error anymore, but it still gives me more than one solution (always the same, the correct one, but repeated). I really cannot understand why.

thanos Over a year ago

can you try this: add to the code foo(_,42). and then run map([1,2,3],foo,X). and map([1],foo,X) it's quite weird, i'm running 5.10.4 too. are you on linux? in any case you can fix it with a wrapper predicate: map_w(X,Y,Z):- once(map(X,Y,Z)).

Aslan986 Over a year ago

Yep, I'm on Linux. Thank you for your help.

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