# Keuringsdienst (van Waarde)

![img](https://git.sr.ht/~teutonicjoe/honeybadger/blob/master/dist/haskell.png) ![img](https://git.sr.ht/~teutonicjoe/honeybadger/blob/master/dist/gnu-gpl-v3.png)

If you know, you know. Data validation in Haskell that is composable, made easy and clean.

Licensed under the GNU General Public License v3 or later.

Data validation rules that are easy to write and serve as documentation of your data as well. Therefore, data validations **SHOULD** live next to your data models.


## Context and motivation

There exist many data validation packages, in Haskell and other languages, but so far I have never found something that was flexible but powerful enough for my needs.

I based myself on `Semigroup` and `Monoid` operations, and attempted to make validation rules that are easy to write, read, compose and maintain.

See an example, from my music website WikiMusic.

    validateEntity x =
      (identifier x) |?| isTextOfLength 36
      <> (displayName x) |?| (isNonEmptyText <> isTextSmallerThanOrEqual 120)

Imagine a simple data model (record) for a music artist:

    data Artist = Artist
    { identifier :: Text,
      displayName :: Text,
      createdBy :: Text,
      visibilityStatus :: Int,
      approvedBy :: Maybe Text,
      createdAt :: UTCTime,
      lastEditedAt :: Maybe UTCTime,
      artworks :: Map Text ArtistArtwork,
      comments :: Map Text ArtistComment,
      opinions :: Map Text ArtistOpinion,
      spotifyUrl :: Maybe Text
    }
    deriving (Eq, Show, Generic)


## Defining validations

Of course for all this to work, you need some imports:

    import Keuringsdienst
    import Keuringsdienst.Helpers

You can define a validation function for `Artist` by composing validation results and rules. Validation results are the result of applying rules to certain data and can be composed with `<>` since they are `Monoid`. Rules can also be composed (`AND`) with `<>` since they are also `Monoid` and can be `OR`'ed with the `*||*` operator, a.k.a `ofDitOfDat`.

    validateArtist :: Artist -> ValidationResult
    validateArtist x =
      (identifier x) |?| isTextOfLength 36
        <> (displayName x) |?| (isNonEmptyText <> isTextSmallerThanOrEqual 120)
        <> (createdBy x) |?| isTextOfLength 36
        <> (visibilityStatus x) |?| isPositiveOrZero
        <> (approvedBy x) |??| isTextOfLength 36
        <> (spotifyUrl x) |??| isNonEmptyText


## What is a ValidationResult

    type ErrMsg = Text
    
    data Validation err
      = Success
      | Failure err
      deriving (Eq, Show, Generic, FromJSON, ToJSON)
    
    type ValidationResult = Validation [ErrMsg]


## Optics / Lenses

If you like `Optics` and lenses as I do, you are fully free to use it:

    validateArtist :: Artist -> ValidationResult
    validateArtist x =
      (x ^. #identifier) |?| isTextOfLength 36
        <> (x ^. #displayName) |?| (isNonEmptyText <> isTextSmallerThanOrEqual 120)
        <> (x ^. #createdBy) |?| isTextOfLength 36
        <> (x ^. #visibilityStatus) |?| isPositiveOrZero
        <> (x ^. #approvedBy) |??| isTextOfLength 36
        <> (x ^. #spotifyUrl) |??| isNonEmptyText