wildthink / Expression

A Mac and iOS library for evaluating numeric expressions at runtime

Covfefe

Covfefe is a parser framework for languages generated by any (deterministic or nondeterministic) context free grammar. It implements the Earley and CYK algorithm.

Usage

Swift Package Manager

This framework can be imported as a Swift Package by adding it as a dependency to the Package.swift file:

Swift 4.1

.package(url: "https://github.com/palle-k/Covfefe.git", majorVersion: 0, minor: 4)

Swift 4.0

.package(url: "https://github.com/palle-k/Covfefe.git", majorVersion: 0, minor: 3)

CocoaPods

Alternatively, it can be added as a dependency via CocoaPods (iOS, tvOS, watchOS and macOS).

Swift 4.1

target 'Your-App-Name' do
  use_frameworks!
  pod 'Covfefe'
end

Swift 4.0

target 'Your-App-Name' do
use_frameworks!
pod 'Covfefe', '0.3.7'
end

Some grammar features are not available when using Swift 4.0 (Grouping, Repetitions, Optional Sequences and Character Ranges, EBNF import)

To add this framework manually:

1. git clone https://github.com/palle-k/Covfefe.git
2. cd Covfefe
3. swift package generate-xcodeproj
4. Drag the generated project file into your project
5. Add Covfefe.framework in the Embedded Binaries section of your project settings

Example

Grammars can be specified in a superset of EBNF or a superset of BNF, which adopts some features of EBNF (documented here).

let grammarString = """
expression       = binary-operation | brackets | unary-operation | number | variable;
brackets         = '(', expression, ')';
binary-operation = expression, binary-operator, expression;
binary-operator  = '+' | '-' | '*' | '/';
unary-operation  = unary-operator, expression;
unary-operator   = '+' | '-';
number           = {digit};
digit            = '0' ... '9';
variable         = {letter};
letter           = 'A' ... 'Z' | 'a' ... 'z';
""" 
let grammar = try Grammar(ebnf: grammarString, start: "expression")

This grammar describes simple mathematical expressions consisting of unary and binary operations and parentheses. A syntax tree can be generated, which describes how a given word was derived from the grammar above:

let parser = EarleyParser(grammar: grammar)

let syntaxTree = try parser.syntaxTree(for: "(a+b)*(-c)")