<<http://i.imgur.com/Ns5hntl.jpg>>
Several datatypes like Either but with differing properties and typeclass instances.
Library support is provided for those different representations, include
lens
related functions for converting between each and abstracting over their similarities.
AccValidation
The AccValidation
data type is isomorphic to Either
, but has an instance of
Applicative
that accumulates on the error side. That is to say, if two (or
more) errors are encountered, they are appended using a Semigroup
operation.
As a consequence of this Applicative
instance, there is no corresponding
Bind
or Monad
instance. AccValidation
is an example of, "An applicative
functor that is not a monad."
Validation
The Validation
data type is isomorphic to Either
and has a Monad
instance
that does the same as Either
. The only difference to Either
is the
constructor names and surrounding library support.
ValidationT
The ValidationT
data type is the monad transformer for Validation
.
An instance of MonadTrans
is provided for (ValidationT err)
. Due to the
arrangement of the ValidationT
type constructor, which permits a MonadTrans instance, there is no possible
Bifunctorinstance. Consequently, the
ValidationBdata type provides a
Bifunctorinstance (but not a
MonadTransinstance). Library support is provided to exploit the isomorphism to
ValidationB`.
Note that since AccValidation
is not a monad, there is also no corresponding
monad transformer for this data type.
ValidationB
The ValidationB
data type is similar to the monad transformer for
Validation
(ValidationT
), however, due to the arrangement of the
ValidationB
type constructor, which permits a Bifunctor
instance, there is
no possible MonadTrans
instance. Consequently, the ValidationT
data type
provides a MonadTrans
instance (but not a Bifunctor
instance).
Library support is provided to exploit the isomorphism to ValidationT
.
Validation'
The Validation' err a
typealias is equivalent to ValidationT err Identity a
and so is isomorphic to Either
and others. Libraries are supplied
accordingly.
Package Version  Update ID  Released  Package Hub Version  Platforms  Subpackages 

0.5.4bp150.1.3 info  GA Release  20180730  15 

