Translating a list to another list in prolog -

i tried write simple code in prolog translate list list. instance, if call listtrans([a,b,c],l), l become [1,2,3]. (a,b,c replaced 1,2,3). faced syntax error in last line. problem? here code:

trans(a,1). trans(b,2). trans(c,3).  listtrans([],l). listtrans([h|t],l1):-    trans(h,b),    append(b,l,l2),    listtrans(t,l2). 

the error because in code:

listtrans([h|t],l1):-    trans(h,b),    append(b,l,l2),    listtrans(t,l2). 

the variable l1 declared in head, not referenced anywhere: mispelled something?

anyway, code not going work.

moreover, using append/3 kind of tasks defined recursion considered terrible (also because of bad performance out of it).

applying function list straightforward. know in prolog don't write y = f(x) rather declare functional relation between x , y as: f(x, y).. (that's did trans(x,y)).

now (easy) recursive formulation:

• the transformed empty list empty list
• the transformation of [x|xs] [y|ys] if trans(x,y) , recursively transform xs ys

or expressed in prolog:

listtrans([],[]). listtrans([x|xs],[y|ys]) :- trans(x,y), listtrans(xs,ys). 

i recommend reading first 4 chapters of learn prolog now better understand these concepts.