phrase/[2,3]phrase(+PhraseType, +List)
phrase(+PhraseType, +List, -Rest)
Used in conjunction with a grammar to parse or generate strings.
Name of a phrase type.
A list of symbols — tokens or codes.
A tail 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, this goal corresponds to using the grammar for parsing. If List is unbound, 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