R/qp.R

In mslegrand/pegr: PEG PARSER

#' A simple tool to examine a parsing expression in isolation
#' 
#' This can be quite useful in getting to know the PEG syntax
#' @param expression, a parsing expression (the right hand side of a rule definiton) 
#' @param text.input, text string to apply expression to.
#' @param record=TRUE, By default TRUE, so one can immediately graph (plot) or print (tree)
#' @return PEGResult, an object giving the result
#' @examples
#' # A simple choice operator
#' qp("'a' / 'b'")( "ab")
#' # A simple sequence operator
#' qp("'a' 'b'")( "ab")
#' # A combination of choice and sequence
#' qp("('a'/'c') ('b' / 'd')")( "ab")
#' qp("('a'/'c') ('b' / 'd')")( "cd")
#' # A lookahead not operator
#' qp("'a' !'b'")( "ab" )
#' qp("'a' !'b'")( "ac" )
#' # An lookahead and operator
#' qp("'a' & 'b'")( "ab")
#' qp("'a' & 'b'")( "ac")
#' # An optional operator
#' qp("'a' 'b'?")( "ab")
#' qp("'a' 'b'?")( "ac")
#' qp("'a'? 'b'")( "ab")
#' qp("'a'? 'b'")( "ba")
#' @export
qp<-function(p.expression, record=TRUE){
  function(text.input){
    if(!("character" %in% class(text.input))){
      stop("qp is the missing text.input", call.=FALSE)
    }
    peg<-new.parser()
    add_rule(peg, paste("R<-", p.expression))->peg
    apply_rule(peg, 'R', text.input, record)->res
    res$Call$rule.id<-"Anonomous"
    return(res)    
  }
}