Sexpr

A ruby compilation framework around s-expressions.

https://github.com/blambeau/sexpr

Features/Problems

  • Provides a YAML format for describing grammars (abstract syntax trees, more precisely).
  • Provides a simple way to check the validity of a s-expression against a given grammar.
  • Provides a framework for processing and rewriting abstract syntax trees.
  • Focusses on the semantic pass, not the syntactic one.
  • Smoothly, yet not tightly, integrates with the Citrus PEG parser (for the syntactic pass).

Example

# Let load a grammar defined in YAML
grammar = SexpGrammar.load("  rules:\n    # alternative rule\n    bool_expr:\n      - bool_and\n      - bool_or\n      - bool_not\n      - var_ref\n      - bool_lit\n\n    # non-terminal\n    bool_and:\n      - [ bool_expr, bool_expr ]\n    bool_or:\n      - [ bool_expr, bool_expr ]\n    bool_not:\n      - [ bool_expr ]\n    bool_lit:\n      - [ truth_value ]\n    var_ref:\n      - [ var_name ]\n\n    # terminals\n    var_name:\n      !ruby/regexp /^[a-z]+$/\n    truth_value:\n      - true\n      - false\n")

Checking the structure of s-expressions

# the grammar can be used to verify the structure of s-expressions
grammar === [:bool_and, [:bool_not, [:var_ref, "x"]], [:bool_lit, true]]
# => true

grammar === [:bool_and, [:bool_lit, "true"]]
# => false (second term is missing)

Including s-expression tools

# the grammar can also be used to automatically have support on top of
# such s-expressions
expr = grammar.sexpr([:bool_lit, true])

Sexpr === expr
# => true

expr.sexpr_type
# => :bool_lit

expr.sexpr_body
# => [true]

# Rewriting s-expressions through copying is easy...
copy = expr.sexpr_copy do |base,child|
  # copy a s-expression ala Enumerable#inject (base is [:bool_lit] initially)
  base << [:bool_lit, !child]
end
# => [:bool_lit, [:bool_lit, false]]

# ... and is tag preserving (including User-included modules)
Sexpr === copy
# true

Have a look at the examples directory.

Public API

sexpr uses Semver and reached 1.0. The public API is as follows:

  • The structure of YAML grammar files
  • The Sexpr module and its public methods
  • The behavior of the Grammar class through its public methods
  • The behavior of the Node class (public methods)
  • The behavior of the Processor and Rewriter classes (public & protected methods)
  • The list of error classes and when they are raised

Contribute

Please use github issues and pull requests for all questions, bug reports, and contributions. Don't hesitate to get in touch with us with an early code spike if you plan to add non trivial features.

Licence

This software is distributed by Enspirit SRL under a MIT Licence. Please contact Bernard Lambeau ([email protected]) with any question.

Enspirit (https://enspirit.be) and Klaro App (https://klaro.cards) are both actively using and contributing to the library.