alternation {qmrparser}

Alternative phrases

Description

Applies parsers until one succeeds or all of them fail.

Usage

  
  alternation(..., 
              action = function(s)   list(type="alternation",value=s), 
              error  = function(p,h) list(type="alternation",pos  =p,h=h) ) 
  

Arguments

...

list of alternative parsers to be executed

action

Function to be executed if recognition succeeds. It takes as input parameters information derived from parsers involved as parameters

error

Function to be executed if recognition does not succeed. I takes two parameters:

• p

  with position where parser, streamParser, starts its recognition, obtained with streamParserPosition

• h

  with information obtained from parsers involved as parameters, normally related with failure(s) position in component parsers.

  Its information depends on how parser involved as parameters are combined and on the error definition in these parsers.

Details

In case of success, action gets the node from the first parse to succeed.

In case of failure, parameter h from error gets a list, with information about failure from all the parsers processed.

Value

Anonymous functions, returning a list.

function(stream) –> list(status,node,stream)

From these input parameters, an anonymous function is constructed. This function admits just one parameter, stream, with streamParser class, and returns a three-field list:

• status

  "ok" or "fail"

• node

  With action or error function output, depending on the case

• stream

  With information about the input, after success or failure in recognition

Examples

# ok
stream  <- streamParserFromString("123 Hello world")
( alternation(numberNatural(),symbolic())(stream) )[c("status","node")]


# fail
stream  <- streamParserFromString("123 Hello world")
( alternation(string(),symbolic())(stream) )[c("status","node")]