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This is a follow-up to a question I asked on Gitter a few days ago.
The basic idea is that it seems useful to be able to abstract over
Validated[E, ?]
as a context in which errors can be raised and handled, butValidated
isn't monadic, so we can't provide aMonadError
instance. None of theMonadError
operations require monadic binding, though, and at least in the case ofValidated[E, ?]
there are reasonable implementations. The onlyMonadError
law we lose is the left zero law.This PR adds an
ApplicativeError
type class aboveMonadError
, together with laws forApplicativeError
and an instance and tests forValidated
. CurrentMonadError
usage should be unaffected.