Question

Hello I have a predicate that returns example equalisation for given list

equalisation([1,2,3,4,12],L).
L = (1=2-3* (4/12)) 

now I want to change this result into a list like this:

L = [1,=,2,-,3,*,4,/,12]

How can I do that?

Answer 1

The simplest solution would be to traverse a term creating a list of small lists, and then to flatten everything. However, the simplest solution is not the most elegant one – quoting the SWI Prolog flatten/2 manual:

Ending up needing flatten/3 often indicates, like append/3 for appending two lists, a bad design. Efficient code that generates lists from generated small lists must use difference lists, often possible through grammar rules for optimal readability.

So here is an alternative solution using difference lists:

t2l(Term, List) :- t2l_(Term, List-X), X = [].

t2l_(Term, [F|X]-X) :- Term =.. [F], !.
t2l_(Term, L1-L4) :- Term =.. [F, A1, A2], 
                     t2l_(A1, L1-L2), 
                     L2 = [F|L3], 
                     t2l_(A2, L3-L4).

Clearly, the current solution works only for binary operations. If operations with an arbitrary number of arguments are allowed, then another traversal of the arguments will be required.