Evaluate term in Prolog and write it - prolog

Question

Hi I am learning prolog (using swi-prolog 7 on Mac) but I cannot find enough docs/tutorials to explain how to print out a result of a term evaluation. I tried code below but it always prints out ERROR: prolog/test.pl:2: user:main: false for any arguments.
#!/usr/bin/env swipl
:- initialization(main, main).
fun1(A, B):- A < B.
fun2([A1 | A2]):- A3 is fun1(A1, A2), write(A3).
main(args) :- fun2(args).
How can I write result of fun1 to stdout in SWI-Prolog?

Answer 1

Perhaps this?
fun1(A,B,Value) :-
(
A < B
->
Value = true
;
Value = false
).
fun2(A1,A2) :-
fun1(A1, A2, Value ),
format('Result: ~w~n',[Value]).
Example run:
?- fun2(1,2).
Result: true
true.
In Prolog you have to think of the result of each line as being true or false and then possibly binding a value to a variable or something more complex as state.
The code in the question was returning that the predicate was true or false but not binding a value to a variable or altering state. By adding Value as an additional parameter and then binding a value in the predicate, the value in Value was able to be used for display.
EDIT
Question from OP in comment
I have never seen -> is it documented somewhere? Sorry if it is a noobie question.
No it is not a noobie question, and actually it was wise of you to just ask instead of festering on it.
See: Control Predicates
In particular ->/2 or (:Condition -> :Action) is often used with ;/2 and together they work like an if then else, e.g.
if then else syntax:
NB This is not Prolog syntax but a syntax common in many imperative programming languages.
if (<condition>) then
<when true>
else
<when false>
-> ; syntax:
This is Prolog syntax.
(
<condition>
->
<when true>
;
<when false>
)
EDIT
Question from OP in comment
When I run this code without the init block and main, so just in interactive mode, then it works. When I try to make a script out of it I get the same error ERROR: prolog/test.pl:2: user:main: false
First
main(args) :- fun2(args).
args is a value but needs to be a variable and in Prolog variables by default start with a capital letter.
main(Args) :- fun2(Args).
Next, Args as received in main/1 is a list, but fun2/2 expects two separate parameters. So by deconstructing Args into a list of two items with Args = [A1,A2] the items in the list can be used as individual items to be passed as parameters to fun2/2.
main(Args) :-
Args = [A1,A2],
fun2(A1,A2).
Example run from top level.
?- main([1,2]).
Result: true
true.
I leave it as an exercise to check if this works as needed from the command line.