validation-selective
Lightweight pure data validation based on Applicative
and Selective
functors.
validation-selective
is built around the following data type:
data Validation e a
= Failure e
| Success a
This data type is similar to Either
but allows accumulating all
errors instead of short-circuiting on the first one.
For more examples and library tutorial, refer to Haddock:
Comparison with other packages
validation-selective
is not the only package that provides such
Validation
data type. However, unlike other packages, it has some
noticeable advantages:
- Lightweight.
validation-selective
depends only on base
and
selective
(which is tiny) Haskell libraries which make this
package fast to build. So adding validation capabilities to your
library or application doesn’t contribute much to your dependency
footprint.
- Selective instance.
validation-selective
is the only package
that provides Selective
instance for Validation
which allows
using Monad
-like branching behaviour but without implementing
wrong Monad
instance.
- More algebraic instances.
validation-selective
also provides
the Alternative
instance and a more general Semigroup
instance.
- Best-in-class documentation. Official Haddock documentation
contains mini-tutorial, usage example, per-component comparison with
Either
, the motivation behind each instance and the interface in
general along with examples for each instance and function.
The below section provides per-package comparison with the most
popular validation packages in the Haskell ecosystem:
either
: Validation
implementation by Edward Kmett. This package is more heavyweight,
since it depends on more Haskell libraries like profunctors
,
bifunctors
, semigroupoids
. But it also provides prisms for
Validation
and some combinators for Either
.
validation
:
Validation
from Queensland Functional Programming Lab.
Depends on lens
, which makes it even heavier but also have richer
interface compared to the either
package.
How to use
validation-selective
is compatible with the latest GHC compiler
versions starting from 8.6
.
In order to start using validation-selective
in your project, you
will need to set it up with the three easy steps:
-
Add the dependency on validation-selective
in your project’s
.cabal
file. For this, you should modify the build-depends
section by adding the name of this library. After the adjustment,
this section could look like this:
build-depends: base ^>= 4.14
, validation-selective ^>= 0.0
-
In the module where you wish to implement pure data validation, you
should add the import:
import Validation (Validation (..))
-
Now you can use the types and functions from the library:
main :: IO ()
main = print [Failure "wrong", Success 42]
Usage with Stack
If validation-selective
is not available on your current Stackage
resolver yet, fear not! You can still use it from Hackage by adding
the following to the extra-deps
section of your stack.yaml
file:
extra-deps:
- validation-selective-CURRENT_VERSION