sweet js ML-style let bindings for auto-curried functions

let.sjs

// Auto curried function definitions using let, like ML
//
// Examples:
// let add x y = x + y
// let map f ctx = tx.map(f)
// NOTE: requires auto-curried function macro: https://gist.github.com/WreckedAvent/9aa698d3e9e6a46c5c62c1e266c95f20
syntax let = ctx => {
  const ids = []
  let ret = null
  let body = null
  
  // start parsing our context
  for (let value of ctx) {
    // we want to capture how many identifiers we have to determine what kind of let we have
    if (value.isIdentifier()) {
      ids.push(value)
    }
    
    if (value.isPunctuator()) {
      // if we run into a punctuator (like =) after we got one identifier, that's a normal let, so 
      // we don't need to parse anything more
      if (ids.length === 1) break
      // otherwise we have more than one identifier, so parse it as a function body
      else { 
        // ctx.mark()
        // var next = ctx.next().value
        // https://github.com/sweet-js/sweet.js/pull/565
        
        // right now, just assume it's an expression - see above pull for when
        // we can lookahead and see if it's also a block
        body = ctx.expand('expr').value
        break
      }
    }
  }
  
  // normal let
  if (ids.length === 1) {
    ctx.reset()
    return #`let ${ctx}` // just pass through normal compilation
  }
  
  // in function let form, the first identifier is the name of our let function
  let [name, ...others] = ids
  
  // function let structure
  return #`let ${name} = fun ${others} { ${body} }`
}