Which Monad Transformer to use?

Question

I am trying to write the validate function below so that the validation stops after the first error encountered. The return type of three is different to the ot

Answer 1

There's nothing much to add to Travis' answer (as usual), but in the case you're using this in a Play! application, maybe https://github.com/Kanaka-io/play-monadic-actions can provide some help.