how to sort the final result in swi-prolog?(predicate included)

Question

I am new to swi-prolog, so basically I have no idea how to write the code... anyway I need to write a precidate about course prerequisite. It will return a list of all courses that are required. And here is my code:

course(cmput325).
course(cmput175).
course(cmput201).
course(cmput204).
prerequisite(cmput204, cmput325).
prerequisite(cmput175, cmput201).
prerequisite(cmput175, cmput204).

/*required(+C,?L)*/

pre(X,C) :- prerequisite(X,C).
pre(X,C) :- prerequisite(X,Y), pre(Y,Z).


pre2(C,L) :-  findall(L1,pre(L1,C),L).

required(C,L) :- sort(pre2(C,L1),L).

I got all correct except the last one required(C,L), pre2(cmput325,L) will returns [cmput204,cmput175], and I want to sort this list, so it will become[cmput175,cmput204]. Then I tried to write a new precidate to deal with it. However, swi-prolog just gave me error:

ERROR: Type error: `list' expected, found `pre2(cmput325,_3718)' (a 
compound)
ERROR: In:
ERROR:    [9] sort(pre2(cmput325,_3770),_3764)
ERROR:    [8] required(cmput325,_3796) at c:/users/mxu3/desktop/a3.pl:19
ERROR:    [7] <user>
Exception: (9) sort(pre2(cmput325, _3322), _3110) ? creep
Exception: (8) required(cmput325, _3110) ? creep

At this stage, I have no idea how to solve this problem... could someone please tell me where should I put this sort command? Thank you....

Consider rolling back your edit and posting everything new ("now I need to write a new ...") as a new question. — repeat, Mar 14 '19 at 19:22
OK, I will delete them and write a new question, thank you for your suggestion. — Code Vanessa, Mar 14 '19 at 19:24

score 3 · Accepted Answer · answered Mar 14 '19 at 09:30

3

You see sort( pre2( C,L1), L) as a "nested expression". You expect its inner expression to be evaluated for value, then that value to be substituted in its place and then the outer expression evaluated with that nested result value used in place of the nested expression. Lisp-like languages work this way. Expression evaluation-oriented languages work this way.

Prolog does not work this way. It does not "evaluate" nested "expressions". Instead, Prolog is written in what reminds one of the SSA (static single assignment) style: instead of

sort( nested( expression, nested_result), result)

write

nested(       expression, nested_result), 
sort(                     nested_result,  result)

minikanren folks call this "unnesting".

answered Mar 14 '19 at 09:30

Will Ness

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Thank you, your answer helps me a lot! – Code Vanessa Mar 14 '19 at 17:21
you're welcome. :) if you have follow-up questions, don't hesitate to ask. – Will Ness Mar 14 '19 at 17:22
Sorry, I actually met another problem, and could you please help me? – Code Vanessa Mar 14 '19 at 18:56

how to sort the final result in swi-prolog?(predicate included)

1 Answers1