Headline

Parsing in Haskell with Parsec

Motivation

The implementation demonstrates parsing in Haskell with the Parsec library of parser combinators. A concrete textual syntax for companies is assumed. Parse trees are constructed in accordance to an abstract syntax defined in terms of algebraic data types. We set up basic parsers for quoted strings and floating-point numbers. Further, we compose parsers for companies, departments, and employees using appropriate parser combinators for sequences, alternatives, and optionality. By design, the parser is kept simple in terms of leveraged programming technique; in particular, monadic style and applicative functors are avoided to the extent possible.

Illustration

See Contribution:haskellAcceptor for a basic illustrationof Parsec-based parsing. The present contribution is more complex in that it constructs proper parse trees; see the the uses of ">>=" for sequential composition with capture of intermediate results and "return" for constructing the parse trees.

parseDepartment :: Parser Department
parseDepartment = Department
  <$> (parseString "department"
  >>  parseLiteral)
  <*  parseString "{"
  <*> parseEmployee "manager"
  <*> many parseSubUnit
  <*  parseString "}"

To this end, we use a parser type that is still parametric in the type of parse trees. Thus:

-- Shorthand for the parser type
type Parser = Parsec String ()

Relationships

Architecture

There are these modules:

  • Main: parser test
  • Company/Parser: the actual parser
  • Company/Data: the abstract syntax definition
  • Company/Sample: a baseline for testing at the level of abstract syntax
The input is parsed from a file "sampleCompany.txt".

Usage

See https://github.com/101companies/101haskell/blob/master/README.md.

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