Remove Ambiguity in abstract syntax in other to write DCG parser Prolog

Question

P => Program K => Block

S => Single-command

C => Commands

E => Expression

B => Boolean-expr

I => Identifier

N > Numeral

Answer 1

Prolog evaluates top down, then LL grammar techniques can be adapted. DCG are more powerful than LL(1), but left recursion must still be eliminated.

B is easier to handle: left factor the production.

B ::= E Bx | not B | (B)
Bx ::= = E | > E | < E | != E | and B | or B

E is harder, because the miss mul token introduces still more ambiguity. Tentatively

E ::= − E | (E) | I | N | El
El ::= Ex E | epsilon
Ex ::= + El | − El | div El | mod El

epsilon (empty production) in DCG can be written this way

epsilon --> [].

If you need to handle precedence and associativity (in both B and E) you will need more work. You can refer to this older answer for a working schema.