Add power to validate and ensure

[FintanH]

It would be really useful if the validate and ensure used (a -> Maybe b) instead of (a -> Bool). This would allow the user to output an updated, validated value as a result.

For example, my current use case is that I'm validating an Integer to a Natural, ensuring that the Integer is (n >= 0).

So I could do:

positive :: Integer -> Validation Errors Natural
positive n = 
  validate (pure $ NegativeVal n) (if n <= then Just $ fromInteger n else Nothing) n

What do you think? :)

[FintanH]

Granted, I could just do an fmap at the end of the computation too

[tonymorris]

Agree. +1