#### 4.14.3 An Example

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]`

. 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

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