readme.md

In chappers/Ramble: Parser Combinator for R

Ramble

This project is just an example to examine the functional components of R.

Installation

Ramble is now on CRAN:

install.packages("Ramble")

The development version can be installed from github:

# install.packages("devtools")
devtools::install_github("chappers/Ramble")

Goals

Create a parser combinator written in pure R. This is mostly a proof of concept, but could be useful or helpful to someone.

This is inspired by Programming in Haskell by Graham Hutton, and also the JavaScript port, and Python's recursive descent parsing library.

References:

• Higher-order functions for parsing

Contributing

You can contribute by opening issues on Github or implementing things yourself and making a pull request.

Please ensure that package passes all checks with --as-cran flag (i.e. via devtools::check(args = c('--as-cran'))) before submitting a pull request.

How it Works

To understand the differences between Ramble and other combinatory parsers please read Ramble: A Parser Combinator in R.

Example

You may view examples for:

• Parsing xml file
• Creating a simple calculator
• Reading a number given in words, and converting it to the appropriate numeric value

Within the examples/* folder. Below is the calculator example.

#' expr :: = term + term | term - term | term
#' term :: = factor * factor | factor / factor | factor
#' factor :: = (expr) | digit+

expr <- ((term %then% 
            symbol("+") %then%
            expr %using% function(x) {
              print(unlist(c(x)))
              return(sum(as.numeric(unlist(c(x))[c(1,3)])))
            }) %alt% 
           (term %then% 
              symbol("-") %then%
              expr %using% function(x) {
                print(unlist(c(x)))
                return(Reduce("-", as.numeric(unlist(c(x))[c(1,3)])))
              }) %alt% term)


term <- ((factor %then% 
             symbol("*") %then%
             term %using% function(x) {
               print(unlist(c(x)))
               return(prod(as.numeric(unlist(c(x))[c(1,3)])))
             }) %alt% 
           (factor %then% 
              symbol("/") %then%
              term %using% function(x) {
                print(unlist(c(x)))
                return(Reduce("/", as.numeric(unlist(c(x))[c(1,3)])))
              }) %alt% factor)

factor <- ((
    symbol("(") %then%
      expr %then%
      symbol(")") %using% 
      function(x){
        print(unlist(c(x)))
        return(as.numeric(unlist(c(x))[2]))
        })
    %alt% natural())

Output:

> expr("(1+1)*2")
[1] "1" "+" "1"
[1] "(" "2" ")"
[1] "2" "*" "2"
[1] "1" "+" "1"
[1] "(" "2" ")"
[1] "2" "*" "2"
[1] "1" "+" "1"
[1] "(" "2" ")"
[1] "2" "*" "2"
$result
[1] 4

$leftover
[1] ""

> expr("(1+2)*3")
[1] "1" "+" "2"
[1] "(" "3" ")"
[1] "3" "*" "3"
[1] "1" "+" "2"
[1] "(" "3" ")"
[1] "3" "*" "3"
[1] "1" "+" "2"
[1] "(" "3" ")"
[1] "3" "*" "3"
$result
[1] 9

$leftover
[1] ""

> expr("1*(2+3)*4*5")
[1] "2" "+" "3"
[1] "(" "5" ")"
[1] "4" "*" "5"
[1] "5"  "*"  "20"
[1] "1"   "*"   "100"
[1] "2" "+" "3"
[1] "(" "5" ")"
[1] "4" "*" "5"
[1] "5"  "*"  "20"
[1] "1"   "*"   "100"
[1] "2" "+" "3"
[1] "(" "5" ")"
[1] "4" "*" "5"
[1] "5"  "*"  "20"
[1] "1"   "*"   "100"
$result
[1] 100

$leftover
[1] ""

> expr("(4-2)+3")
[1] "4" "-" "2"
[1] "(" "2" ")"
[1] "4" "-" "2"
[1] "(" "2" ")"
[1] "4" "-" "2"
[1] "(" "2" ")"
[1] "2" "+" "3"
$result
[1] 5

$leftover
[1] ""

chappers/Ramble documentation built on May 13, 2019, 3:30 p.m.