Dynamic object validation using $ObjMap in Flow

validation.flow.js

/* @flow */

// A simplified representation of types using phantom types (so that we store the Type information both at value and type level)
class Type<T> {};
class StringT extends Type<string> {}
class NumberT extends Type<number> {}

// A schema for a User
const User = {
  name: new StringT(),
  age: new NumberT()
};

// function that checks whether a value conforms to a type representation
// this is a trivial implementation that uses instance checking
function isValid<V>(value: V, type: Type<any>): boolean {
  if (type instanceof StringT) {
    return typeof value === 'string';
  } else if (type instanceof NumberT) {
    return typeof value === 'number';
  }
  return false;
}

// a function that validates a JSON string against a schema
// the result type is either an Error or a transformation over the schema, where
// the values have been replaced with the actual values of the object.
// e.g. { age: 'number' } is mapped to { age: 42 }
function validate<O: { [_: string]: Type<any> }>(schema: O, value: string): Error | $ObjMap<O, <V>(_: Type<V>) => V> {
  try {
    const parsed = JSON.parse(value);

    // for each key of the parsed object, check whether the type matches the expectations
    Object.keys(parsed).forEach(k => {
      const value = parsed[k];
      const expected = schema[k];
      if (!isValid(value, expected)) {
        throw TypeError(`InvalidSchema. Expected ${k} of type ${expected}, got ${value} of type ${typeof value}`);
      }
    });

    // return the validated object
    return parsed;
  } catch (e) {
    // reify the exception, returning it instead of throwing
    return e;
  }
}

const ok = validate(User, '{ "name": "gabro", "age": 42 }');
const wrongSyntax = validate(User, '{ name": "gabro", "age": 42 }');
const wrongType = validate(User, '{ "name": "gabro", "age": true }');

printOrError(ok);          // 42
printOrError(wrongSyntax); // Invalid Syntax ...
printOrError(wrongType);   // Invalid Schema ...

function printOrError(result) {
  // Note that the instance check is crucial. The return type can be a validated object or an Error, so something like
  //   console.log(result.age)
  // would result in type error, since we don't know yet whether it's one or the other.
  if (result instanceof Error) {
    const error = result;
    console.log(error.message);
  } else {
    const { age, name } = result;
    console.log(`${name} is ${age} years old`);

    // also note that 'name' has type string and 'age' has type 'number'
    // This won't typecheck
    //   console.log(age.trim())
  }
}

Typechecks with flow v0.33+.

Run with flow-node to get the output.

This was a really helpful read–I've learned a lot from reading your and @gcanti's writing. Cheers!