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`phrase/[2,3]`

`phrase(`

`+PhraseType`, `+List`)

`phrase(`

`+PhraseType`, `+List`, `-Rest`)

Used in conjunction with a grammar to parse or generate strings.

`:PhraseType`*callable*, must be nonvarName of a phrase type.

`List`*list of term*A list of symbols — tokens or

*codes*.`Rest`*list of term*A suffix of

`List`; what remains of`List`after`PhraseType`has been found. Defaults to`[]`

.

This predicate is a convenient way to start execution of grammar
rules. Runs through the grammar rules checking whether there is a
path by which `PhraseType` can be rewritten as `List`.

If `List` is bound, then this goal corresponds
to using the grammar for parsing. If `List` is unbound, then this
goal corresponds to using the grammar for generation.

`phrase/[2,3]`

succeeds when the portion of `List` between the start of
`List` and the start of `Rest` is a phrase of type
`PhraseType` (according to the current grammar rules), where
`PhraseType` is either a non-terminal or, more generally, a grammar
rule body.

`phrase/[2,3]`

allows variables to occur as non-terminals in grammar
rule bodies, just as `call/1`

allows variables to occur as goals
in clause bodies.

Depends on `PhraseType`.

Call errors (see ref-sem-exc).

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:

| ?-[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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