Convert a function to a FaunaDB Let expression

functionToLet.js

const { query: q } = require('faunadb')

// fn-annotate
// https://www.npmjs.com/package/fn-annotate
const annotate = require('fn-annotate')

// converts a function into a faunaDB Let expression, but allows it to be used,
// as a regular function.  This allows functions to be composed in JS land and
// turn out correctly in Fauna land.
const functionToLet = (fn) => (...args) => {
  const params = annotate(fn)
  const arity = params.length
  const bindings = args.reduce(
    (result, arg, index) =>
      index < arity
        ? [
            ...result,
            {
              [params[index]]: arg,
            },
          ]
        : result,
    []
  )

  return q.Let(
    bindings,
    fn.apply(
      null,
      params.map((param) => q.Var(param))
    )
  )
}

// EXAMPLE
// *****************************************************************************
// some contrived library functions
const addUniqueField = functionToLet((collectionRef, fieldName) =>
  q.CreateIndex({
    name: 'needsABetterName',
    source: collectionRef,
    unique: true,
    terms: [{ field: ['data', name] }],
  })
)

const refOrAbort = functionToLet((maybeRef) =>
  q.If(q.Exists(maybeRef), maybeRef, q.Abort('does not exist'))
)

// *****************************************************************************
// contrived app code
const collectionRef = refOrAbort(q.Collection('User'))
const query = addUniqueField(collectionRef, 'email')

// *****************************************************************************
// equivalent query
const equivQuery = q.Let(
  {
    collectionRef: q.Let(
      { 
        maybeRef: q.Collection('User')
      },
      q.If(
        q.Exists(q.Var('maybeRef')),
        q.Var('maybeRef'),
        q.Abort('does not exist')
      )
    ),
  },
  q.CreateIndex({
    name: 'needsABetterName',
    source: q.Var('collectionRef'),
    unique: true,
    terms: [{ field: ['data', 'email'] }],
  })
)

module.exports = functionToLet