Thoughts on a decoder "code generator" given a flow type.

One pain point is the decoder is more-or-less just the flowtype rewritten. The advantage of something like https://github.com/codemix/flow-runtime is that this is done automatically, but without the flexibility of making more complex runtime decoders and/or deciding where to apply the runtime check.

What are your thoughts about a tool that would generate decoders for all exported types? For example if you have a file ./src/myModule.js and you run $ decoder-codegen ./src/, it would create a new file ./src/__generated__/myModule.decoders.js. Then we can import the decoders into the source file.

myModule.js

import { guard } from 'decoders';
import { pointDecoder } from './__generated__/myModule.decoders';

const checkPointType = guard(pointDecoder);

export type Point = {
  x: number,
  y : number
}

export function moveUp(point: Point) {
  checkPointType(point);
  point.y = point.y - 1;
}

Inspired by https://github.com/apollographql/apollo-codegen#generate

[sfarthin]

Guard satisfies the majority of my use cases and does not require understanding of decoders to use, though it would be nice in certain situations.

The lib could provide both:

import guard from "flow-guard/macro";
import createDecoder from "flow-guard/decoder-macro";

Additionally, I am thinking decoders that go beyond flow types (integer, regex, email, etc), it might make more sense to have that as an additional optional argument that would “extend” the decoder. The implementation of pulling this from comments or somewhere else would be tricky and maybe a little too magical.

import { email } from 'decoders';
import guard from "flow-guard/macro";

type Person = {
  name: string,
  email: string
}

function personGuard(person: any): Person {
  return guard<Person>(person, {
    email
  });
}

or with decoders…

import { guard, email } from 'decoders';
import createDecoder from "flow-guard/decoder-macro";

type Person = {
  name: string,
  email: string
}

const personDecoder = createDecoder<Person>({
  email
});

const personGuard = guard(personDecoder);

[sfarthin]

fyi, proof of concept started: https://github.com/sfarthin/flow-guard . Might be some time translating the flow AST into decoder code. (https://github.com/sfarthin/flow-guard/blob/master/macro.js#L5)