How to aggregate all success/failures in list of Validation (sequence?)

[OliverJAsh]

Hi, thanks for the fantastic library and learning resources.

I have Array<Validation<A, B>> and want to aggregate all the successes and failures, e.g. Array<Validation<A, B>> => Validation<Array<A>, Array<B>>.

I thought this was possible using control.monads.sequence, however I seem to get runtime error mr.chain is not a function, so I presume I must be doing something wrong.

I have had success sequencing a list of Eithers, but in my case I want to aggregate the left hand side as well, which from reading this blog post, it appears Validation should help.

Thanks!

[OliverJAsh]

Never mind, got it now!

// Array<Validation<A, B>> => Validation<Array<A>, Array<B>>
const sequenceValidations = validations => (
    validations
        .reduce((acc, validation) => (
            Validation.Success(a => b => a.concat(b)).ap(acc).ap(validation)
        ), Validation.of([]))
)

Is this function in the library anywhere?

[vendethiel]

Sounds a bit like http://stackoverflow.com/questions/39227232/futurelisterror-double-to-futurelisterror-listdouble-in-scala/39227649#39227649

[robotlolita]

@OliverJAsh Validations implement Applicative, rather than Monad, so sequence (which uses the Monad's interface .chain) doesn't work on them.

There isn't a function to aggregate Validations yet. Validations themselves already aggregate failures when you use .apply(), but the failure needs to have a .concat method. You can write a function to do this as:

const Validation = require('folktale/data/validation');
const curry = require('folktale/core/lambda/curry');

const intoArray = (x) => Array.isArray(x) ? x : [x];

const collect = (validations) =>
  validations.map(x => x.failureMap(intoArray)).reduce(
    (a, b) => a.ap(b),
    Validation.of(curry(validations.length, (...values) => values))
  );

So something like:

collect([a, b, c]);

Translates to:

Validation.of((a, b, c) => [a, b, c])
  .ap(a.failureMap(intoArray))
    .ap(b.failureMap(intoArray))
      .ap(c.failureMap(intoArray))