index.js

import { default as gram } from 'easygram'

export const read = gram(`
jam          ::= obj | arr | str
obj          ::= WS* '{' WS* (duo (WS* duo)*)? WS* '}' WS*
arr          ::= WS* '[' WS* (jam (WS* jam)*)? WS* ']' WS*
duo          ::= str WS+ jam
str          ::= bare  | WS* '"' quote* '"' WS*
bare         ::= SAFE+
quote        ::= ANY*
WS           ::= [ \t\n\r]+
SYN          ::= '{' | '}' | '[' | ']'
ANY          ::= (SAFE | WS | SYN | #x5C)
SAFE         ::= #x21 | [#x23-#x5A] | [#x5E-#x7A] | #x7C | #x7E
`)

// Expects a JAMS string, the leaves/strings within must be valid JSON strings.
export const jams =s=> {
    const ast = read(s)
    if (ast === null) throw new Error('Syntax error')
    if (ast.errors.length > 0) throw ast.errors[0]
    return _jams(ast)
}

const _jams =ast=> {
    switch (ast.type) {
        case 'jam': {
            return _jams(ast.children[0])
        }
        case 'str': {
            return (ast.text.includes('""') && ast.children.length === 0) ? "" : _jams(ast.children[0])
        }
        case 'bare':
        case 'quote': {
            const quoted = String.raw`"${ast.text}"`
            const json = JSON.parse(quoted)
            return String.raw`${json}`
        }
        case 'arr': {
            return ast.children.map(child => _jams(child))
        }
        case 'obj': {
            const out = {}
            for (let duo of ast.children) {
                const key = _jams(duo.children[0])
                const val = _jams(duo.children[1])
                if (out[key] !== undefined) throw new Error(`Parse error: duplicate keys are prohibited at parse time`)
                out[key] = val
            }
            return out
        }
    }
    throw new Error(`panic: unrecognized AST node ${ast.type}`)
}