Simple ParserCombinator framework for Swift
Operators:
a || b:alternate(a,b)- if left side fails, then consider right side (also known as<|>a + b:then(a,b)- make a tuple from the left side and the right side_ *> b:xthen(_,b)– skip the left side (for example a required keyword) and return the right sidea <* _:thenx(a,_)– skip the left side and return the right sidea => f: apply function to a result- unary
§x:expect(value)– take a symbol matching value - unary
%x:item(expeted)– take any symbol, if does not match then retursn an errorexpected
Here is a simple arithmetic expression parser example. We are using a stream that is already processed. Writing a lexer is left as an exercise to the reader.
import ParserCombinator
// Basic token parser – converts a symbol from input stream into an integer
let number: Parser<String, Int> = %"number" => { num in Int(num)! }
// Declaration for the recursive reference
let expr: Parser<String, Int>
// Here we reference future value of expn
let factor =
(§"(" *> wrap { expr } <* §")")
|| number
let term =
(factor + (§"*" *> factor)) => (*)
|| (factor + (§"/" *> factor)) => (/)
|| factor
// Definition of the
expr =
(term + (§"+" *> term)) => (+)
|| (term + (§"-" *> term)) => (-)
|| term
let parser = expr
let source = ["1", "+", "2", "*", "(", "3", "+","4", ")"].stream()
let result = parser.parse(source)[Higher-Order Functions for Parsing, Graham Hutton] (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.63.3555&rep=rep1&type=pdf)
Write You a Haskell, Stephen Diehl
Stefan Urbanek stefan.urbanek@gmail.com Twitter: @Stiivi
MIT. See the LICENSE file.