sub-subset-.pegR: Reset an existing rule: definition, description, action

In mslegrand/pegr: PEG PARSER

Description

Used to modify the rule definition and/or description and/or action of of an exiting rule

Usage

## S3 replacement method for class 'pegR'
parser[[rule.id]] <- value

Arguments

parser,	the peg parser containing the rule to be modified
rule.id,	the identifier of the rule to be modified
value,	the modifications to be applied. Can be either a named vector or a named list (see below), a unnamed character vector(see below), or NULL, which case the rule is deleted.

Details

This is a very flexiable way of resetting the action, description and even the definition of an existing rule. This encompasses the functionality of set_definition, set_description and set_action This rule accepts either named vectors/lists or unnamed vectors. In both cases, the vectors are character vectors and the list components are either strings or a function representing an action.

Value

peg parser

Using Named Vector

For named vectors the valid names are:

• rule: The name rule is used to specify a field containing a rule definition (a.k.a. source). (like "A<-'a'") to be associated with the named rule.

• des: The name des is used to specify a field containing a description or comment to be associated with the named rule

• act: The name act is used to specify a field containing an action to be associated with the rule. The action can either be NULL, or text which representing the body of function taking a single parameter v that is a list and returns a list. When NULL, any existing action is deleted.

Using Unnamed Vectors

For unnamed vectors, each vector consists of a single string of the form:

• "...<-...": Specifies a rule definition (a.k.a. source). (like "A<-'a'")

• "#..." Specifies description or comment to be associated with the rule (like "# my clever rule")

• "{...} " Specifies an inline action to be associated with the rule. (like "list(toupper(unlist(v)))" )

• "{}=NULL" Specifies to delete any existing to be associated with the rule.

Inline Action Short Cuts

As a convenience the following short cuts are provided:

• "{}": Specifies the action "{list()}", which essentially says drop (ignore) the values produce by this rule.

• "{-}" Specifies the action "list(paste(v,collapse=''))", which says paste all the values together.

Examples

# Modfiying a rule (adding a comment and  action):
peg<-new.parser()
peg + "A<-'a'"
peg
peg[["A"]]<-"A<-'ab'"
peg
peg[["A"]]<-c("A<-'xx'", des="replace xx by a", act="list('a')")

# Another way of doing the same thing (adding a comment and  action):
peg<-new.parser()
peg + "A<-'a'"
peg
peg[["A"]]<-"A<-'ab'"
peg
peg[["A"]]<-c("A<-'xx'", "#replace xx by a", "{list('a')}")

# The following are equivalent:
peg[['A']]<-list(act="list()")
peg[['A']]<-"{}" # A shortcut

# Deleteing a rule
peg<-new.parser()
peg + "A<-'a'" + "B<-'b'"
peg[["A"]]<-NULL
peg

mslegrand/pegr documentation built on May 23, 2019, 7:53 a.m.