Example usage about covfefe

[palle-k]

Hi!

The Earley parser returns a syntax tree with the non-terminals as nodes and the ranges, in which terminals were identified, as leafs:

let tree: SyntaxTree<NonTerminal, Range<String.Index>>

Each value of this type SyntaxTree<NonTerminal, Range<String.Index>> is therefore either a case node(...) or a case leaf(...) and you can process them recursively through pattern matching:

switch tree {
case .node(key: let nonTerminal, children: let subtrees) where nonTerminal.name == "expression":
    // parse whole expression

case .node(key: let nonTerminal, children: let subtrees) where nonTerminal.name == "binary-operation":
    // parse binary operation
    let operatorSymbol = getOperator(subtrees[1]) // the 0th subtree is the left hand side expression, the 1st subtree is the binary-operator, the 2nd subtree is the right hand side expression.

// more cases for the remaining non-terminals
...

case .leaf(let range):
    // range is the position in which this leaf was identified in the parsed string.
    // you can get the text at this position using
    let leafText = String(parsedExpression[range])
}

This forms the base on which you can process your tree, for example mapping it into an expression type or evaluating it.

I have implemented an example project, which evaluates math expressions here: https://github.com/palle-k/ExpressionSolver

Let me know if this helps or if you have any further questions :)

[mlajtos]

Yup. This is it, now I can hack together something. Thank you. ☺️

I added #6 – I think it should be mentioned somewhere because it is a great starting point for anybody that is dumb like me. :)