Understanding a prolog predicate

0
votes

I'm having some troubles in understanding the following prolog predicate , I can understand that it concatenate some chars , also produces the possible lists (first & second goal ) , but I can't understand how it do this ? how it executes ?

domains 

i=integer
l=i*
slist=string*
clist=char*

predicates 

nondeterm conc(clist,clist,clist). 

clauses 

conc([],L,L). 
conc([H|L1],L2,[H|L3]):- 
conc(L1,L2,L3).

The first goal

goal 

conc(['a','b'],['c','d'],L).

result 

L=['a','b','c','d']

the second goal 

goal

conc(L1,L2,['a','b','c']).

result 

L1=[], L2=['a','b','c']
L1=['a'], L2=['b','c']
L1=['a','b'], L2=['c']
L1=['a','b','c'], L2=[]
4 Solutions
1
prologturbo-prolog

1 Answers

3
votes

I'm neither an expert in prolog, nor in logic, but I'll try to explain what I think how it works.

After the call to:

<- conc(['a','b'],['c','d'],L).

Prolog will look for a predicate that matches the signature. In this case that would be:

conc([H|L1],L2,[H|L3])

It tries to resolve the variables with the given data.

 |1: H:=['a'], L1:=['b'], L2:=['c','d']

Now it steps into the recursion with these data calling:

<- conc(['b'], ['c','d'], L3).
 |2: H:=['b'], L1:=[], L2:=['c','d']
<- conc([], ['c','d'], L3).

The last line causes prolog to use the predicate with the signature:

conc([],L,L).

resolving:

 |3: L:=['c','d']

Now Prolog is able to construct the concatinated List handing over L up the recursion stack.

 |2: [H|L3]:=['b','c','d']
 |1: [H|L3]:=['a','b','c','d']

I hope that is a hint in the right direction. Maybe you should read this article for clarification