CODEWITHSUNDEEP
haskellexceptionmonadseither
user6023611
user6023611

Reputation:

Accumulating errors using ErrorT

Is it possible to accumulate error messages in the ErrorT monad? I would like to accumulate more that one error.

Upvotes: 2

Views: 651

Answers (1)

danidiaz
danidiaz

Reputation: 27776

You can use the Errors Applicative from Control.Applicative.Lift from transformers:

ghci> import Control.Applicative
ghci> import Control.Applicative.Lift
ghci> failure ['a'] *> pure () <* failure ['b']
Other (Constant "ab")

It returns the list of errors, if there are any, or the successful result.

This type is usually known as the "Validation" Applicative. There are other implementations of it available on Hackage. One posible improvement is to relax the requirement for the failure container to be a Monoid, allowing Semigroups as well.

Notice that the Errors type is not a Monad. But you can combine it with other Applicatives using Data.Functor.Compose.

The MonadPlus instance for ExceptT has a related but not identical behaviour: it returns the first success, if there is any, or a list of errors:

ghci> throwE ['a'] `mplus` return () `mplus` throwE ['b'] :: ExceptT [Char] Identity ()
ExceptT (Identity (Right ()))
ghci> throwE ['a'] `mplus` throwE ['b'] :: ExceptT [Char] Identity ()
ExceptT (Identity (Left "ab"))

Upvotes: 9

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