Question about usage and need

[fernandocanizo]

Excuse me but I'm a little bit puzzled about the need of tcomb-validation, what's the gain of this:

const tv = require('tcomb-validation');

tv.validate(1, tv.String).isValid();   // => false
tv.validate('a', tv.String).isValid(); // => true

Versus this:

const t = require('tcomb');

const aString = t.String(1);
const bString = t.String('a');

Wasn't the whole point of tcomb to have an identity function which did validation in the background?

With tcomb-validation now I have to add boilerplate code to check every single thing.

Please, don't misunderstand me, I'm not trolling, it's I'm not seeing the benefit, but since someone took the time to make tcomb-validation, then I guess I'm missing something.

[gcanti]

Old but related https://github.com/gcanti/tcomb-validation/issues/3#issuecomment-55791609

TLDR tcomb just throws and is meant to drive out bugs, tcomb-validation returns a list of errors and is meant to validate input

(tcomb can be derived from tcomb-validation but at that time I wrote tcomb first)

[fernandocanizo]

Hello Giulio,

So I think you may start a FAQ now :stuck_out_tongue_closed_eyes: ... Or maybe just state the difference explained in your answer to Brian in a prominent place, in both projects.

I'm traversing the path backwards than Brian: I'd like to drop Joi and use tcomb for everything.

So let me see if I understood this correctly:

• I should use tcomb to make my code type aware, kind of TypeScript type checking. Things like:

const foo = (data) => { const name = t.String(data.name); // ... };

So if a thing that's not a String reaches data.nameit will throw.

• I should use tcomb-validation just for checking user input.

Am I correct?

P.S.: also I'd like to suggest that you use tv to hold tcomb-validation and t just for tcomb, maybe the sample code made it look like the same thing in my head.

[gcanti]

Yes, you nailed it. tcomb-validation just augments tcomb's export with the validate function, runtime types are the same.

Speaking of TypeScript you may also want to take a look at io-ts. It's basically a modern tcomb-validation, compatible with TypeScript. There's no assert function, but is pretty easy to define

import * as t from 'io-ts'
import { PathReporter } from 'io-ts/lib/PathReporter'

function assert<T>(value: any, type: t.Type<T>): T {
  const validation = t.validate(value, type)
  return validation.fold(() => {
    throw new Error(PathReporter.report(validation).join('\n'))
  }, x => x)
}

assert(1, t.number)
assert(1, t.string) // throws "Invalid value 1 supplied to : string"