nasherm / bus-hs

A Haskell parser combinator library

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                                           A parser library
                                 Author:Nashe Mncube/nashpotato

Introduction

BUS is a fully functional parser combinator library written in a Haskell. It exists not to replace current parser libraries, but rather to act as a toy parser library for the curious and learning. Relatively small, bus uses advanced concepts in functional programming, which are easily accessible to a learner of functional languages who may not know these concepts, or may want to see these concepts in action.

How does it work?

The best way to illustrate how this library works is via an example. Suppose that I define a simple language as such which describes a simple language consisting of expressions which can be added, and subtracted.

  <expr>  := ("+"|"-") <term> ( ("+"|"-") <term> )*
  <term>  := ("0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9")+

For those not familiar with Backus-Naur form, this is a way of representing context-free grammars. terms are made of one or strings of separated by plus and minus terms. Each term is simply a string of one or more digits. So for example "1", "-1+2", "42-32" are all strings within the grammar but strings such as 100++, --5, 2-4+ are not.

So what would a parser for this language look like? Well first we would define a data structure that corresponds to our grammar's description.

  data Parity = (:+:) | (:-:) deriving (Show)
  data Term = Term Int deriving (Show)
  data Expr = Expr Parity Term [Expr] deriving (Show)

Then a parser for this language is as simple as the following

  parity :: Parser Parity
  parity =  ((:+:) <$ char '+' <* whitespace )
        <|> ((:-:) <$ char  '-' <* whitespace)

  term :: Parser Term
  term = Term <$> number <* whitespace

  expr :: Parser Expr
  expr = Expr <$> parity <*> term <*> many expr <* whitespace

We can use test the expr parser as so in ghci

  *Main > parse expr "+1 -2"
  *Main > [("",Expr (:+:) (Term 1) [Expr (:-:) (Term 2) []]),("-2",Expr (:+:) (Term 1) []),("",Expr (:+:) (Term 1) [Expr (:-:) (Term 2) []]), ...

Build

To build the library just type make into the terminal