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As an example, here is a simple grammar that parses an arithmetic expression (made up of digits and operators) and computes its value. Create a file containing the following rules:
grammar.pl
expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.
expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.
expr(X) --> term(X).
term(Z) --> number(X), "*", term(Y), {Z is X * Y}.
term(Z) --> number(X), "/", term(Y), {Z is X / Y}.
term(Z) --> number(Z).
number(C) --> "+", number(C).
number(C) --> "-", number(X), {C is -X}.
number(X) --> [C], {"0"=<C, C=<"9", X is C - "0"}.
In the last rule, C is the character code of a decimal digit.
This grammar can now be used to parse and evaluate an expression by means
of the built-in predicates phrase/[2,3].
See mpg-ref-phrase. For example,
| ?- [grammar]. | ?- phrase(expr(Z), "-2+3*5+1"). Z = 14 | ?- phrase(expr(Z), "-2+3*5", Rest). Z = 13, Rest = [] ; Z = 1, Rest = "*5" ; Z = -2, Rest = "+3*5" ; no