valida-base: Simple applicative validation for product types, batteries included!

[ data, library, mit, validation ] [ Propose Tags ]

This is a zero dependency version of the valida package. It is equivalent to valida == 0.1.0. Check out the README at github!

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Versions [RSS] 0.1.0, 0.1.1, 0.2.0
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Dependencies base (>=4.12 && <5) [details]
License MIT
Copyright TotallyNotChase
Author Chase
Category Validation, Data
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Source repo head: git clone
Uploaded by TotallyNotChase at 2021-08-25T07:11:10Z
Distributions LTSHaskell:0.2.0, NixOS:0.2.0, Stackage:0.2.0
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Readme for valida-base-0.2.0

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Simple, elegant, applicative validation for product types - batteries included!

This is a dependency-less version of valida.

Read the documentation on hackage.


  • Minimal - Zero dependencies apart from base.
  • Batteries included - ValidationRule combinators for almost every scenario.
  • Validation without the boiler plate - a highly specialized usage of contravariant functors to conveniently model the common validation usecases, without extra boilerplate.

Quick Taste

import Data.List.NonEmpty (NonEmpty)

import Valida

data InputForm = InpForm
  { inpName  :: String
  , inpAge   :: Int
  , inpEmail :: Maybe String
  } deriving (Show)

data ValidInput = ValidInput
  { vInpName  :: String
  , vInpAge   :: Int
  , vInpEmail :: Maybe String
  } deriving (Show)

data FormErr
  = InvalidNameLength
  | InvalidAge
  | NoAtCharInMail
  | NoPeriodInMail
  | InvalidEmailLength
  deriving (Show)

-- | Validator for each field in the input form - built using 'ValidationRule' combinators.
inpFormValidator :: Validator (NonEmpty FormErr) InputForm ValidInput
inpFormValidator = ValidInput
    -- Name should be between 1 and 20 characters long
    <$> inpName -?> lengthWithin (1, 20) InvalidNameLength
    -- Age should be between 18 and 120
    <*> inpAge -?> valueWithin (18, 120) InvalidAge
    -- Email, if provided, should contain '@', and '.', and be atleast 5 characters long
    <*> inpEmail -?> optionally (minLengthOf 5 InvalidEmailLength
        <> mustContain '@' NoAtCharInMail
        <> mustContain '.' NoPeriodInMail)

goodInput :: InputForm
goodInput = InpForm "John Doe" 42 Nothing

badInput :: InputForm
badInput = InpForm "John Doe" 17 (Just "@")

main :: IO ()
main = do
    print (runForm inpFormValidator goodInput)
    -- Prints- Success (ValidInput {vInpName = "John Doe", vInpAge = 42, vInpEmail = Nothing})
    print (runForm inpFormValidator badInput)
    -- Prints- Failure (InvalidAge :| [InvalidEmailLength])

You can also find more examples here.

Quick Start

The primary purpose of the Validator type is to validate each field in product types. To do this, you'll use verify.

verify takes 2 inputs-

  • The "selector", which essentially just takes the product type as input, and returns the specific value of the specific field to validate.
  • The ValidationRule, which specifies the predicate the field must satisfy - as well as the error value to yield if it doesn't satisfy said predicate

Let's validate a pair for example, the first field should be an int less than 10, the second field should be a non empty string. Then, the validator would look like-

pairValidator :: Validator (NonEmpty String) (Int, String) (Int, String)
pairValidator = (,) <$> verify (failureIf (>=10) "NotLessThan10") fst <*> verify (notEmpty "EmptyString") snd

Or, if you prefer using operators - you can use -?>, which is a flipped version of verify.

pairValidator :: Validator (NonEmpty String) (Int, String) (Int, String)
pairValidator = (,)
    <$> fst -?> failureUnless (<10) "NotLessThan10"
    <*> snd -?> notEmpty "EmptyString"

You can then run the validator on your input using runValidator-

>>> runValidator pairValidator (9, "foo")
Success (9,"foo")
>>> runValidator pairValidator (10, "")
Failure ("NotLessThan10" :| ["EmptyString"])
>>> runValidator pairValidator (5, "")
Failure ("EmptyString" :| [])

This is the core concept for building the validators. You can use the primitive combinators (e.g failureIf, failureUnless) to build ValidationRules directly from predicate functions, or you can choose one of the many derivate combinators (e.g notEmpty) to build ValidationRules. Check out the Valida.Combinators module documentation to view all the included combinators.

Validators for non product types

Although the primary purpose of Valida is building convenient validators for product types. Sometimes, you'll find yourself not needing to select on any field, but validating the input directly. In that case, you may find yourself using this pattern-

-- | Make sure int input is not even.
intValidator :: Validator (NonEmpty String) Int Int
intValidator = verify (failureIf even "Even") id

In these situations, instead of using verify with id as selector, you should use validate instead, which is the same as flip verify id-

intValidator :: Validator (NonEmpty String) Int Int
intValidator = validate (failureIf even "Even")

Combining multiple ValidationRules

Often, you'll find yourself in situations where you expect the input to satisfy multiple ValidationRules, or situations where you expect the input to satisfy at least one of many ValidationRules. This is where andAlso, and orElse come into play.

Combining multiple ValidationRules with andAlso

andAlso is the semigroup implementation of ValidationRule, and thus is the same as <>. Combining 2 rules with <> creates a new rule that is only satisfied when both of the given rules are satisfied. Otherwise, the very first (left most) failure value is returned - and the rest are not tried.

The following rule only succeeds if the input is odd, and not divisble by 3.

rule :: ValidationRule (NonEmpty String) Int
rule = failureIf even "IsEven" `andAlso` failureIf ((==0) . flip mod 3) "IsDivisbleBy3"


rule :: ValidationRule (NonEmpty String) Int
rule = failureIf even "IsEven" <> failureIf ((==0) . flip mod 3) "IsDivisbleBy3"


>>> runValidator (validate rule) 5
Success 5
>>> runValidator (validate rule) 4
Failure ("IsEven" :| [])
>>> runValidator (validate rule) 15
Failure ("IsDivisbleBy3" :| [])
>>> runValidator (validate rule) 6
Failure ("IsEven" :| [])

Combining multiple ValidationRules with orElse

orElse also forms a semigroup, </> is aliased to orElse. Combining 2 rules with </> creates a new rule that is satisfied when either of the given rules are satsified. If all of them fail, the Failure values are accumulated.

The following rule succeeds if the input is either odd, or not divisble by 3.

rule :: ValidationRule (NonEmpty String) Int
rule = failureIf even "IsEven" `orElse` failureIf ((==0) . flip mod 3) "IsDivisbleBy3"


rule :: ValidationRule (NonEmpty String) Int
rule = failureIf even "IsEven" </> failureIf ((==0) . flip mod 3) "IsDivisbleBy3"


>>> runValidator (validate rule) 5
Success 5
>>> runValidator (validate rule) 4
Success 4
>>> runValidator (validate rule) 15
Success 15
>>> runValidator (validate rule) 6
Failure ("IsEven" :| ["IsDivisbleBy3"])

Combining a foldable of ValidationRules

You can combine a foldable of ValidationRules using satisfyAll and satisfyAny. satisfyAll folds using andAlso/<>, while satisfyAny folds using orElse/</>.

Ignoring errors

Although, highly inadvisable and generally not useful in serious code, you may use alternative versions of ValidationRule combinators that use () (unit) as its error type so you don't have to supply error values. For example, failureIf' does not require an error value to be supplied. In case of failure, it simply yields Failure ().

>>> runValidator (validate (failureIf' even)) 2
Failure ()

Re-assigning errors

Using the label/<?> and labelV/<??> functions, you can use override the errors ValidationRules and Validators yield.

For example, to re assign the error on a ValidationRule-

label "IsEven" (failureIf even "Foo")


failureIf even "Foo" <?> "IsEven"

This is useful with ValidationRules that use unit as their error type. You can create a ValidationRule, skip assigning an error to it - and label a specific error when you need it later.

label "IsEven" (failureIf' even)

Re-labeled ValidationRules will yield the newly assigned error value when the rule is not satisfied.

Similarly, labelV (or <??>) can be used to relabel the error value of an entire Validator.

Wait, couldn't this be done using contravariant functors?

Yes! The concept of keeping the input of a Validator set to the same product type, but letting it validate a specific field of said input, can be generalized to contravariant functors. The Validator type looks like- Validator e inp a, to keep applicative composition working, the inp needs to stay the same - but each validator within said composition should also be able to consume a specific part of the inp. ValidationRule itself, is the typical example of a contravariant functor as well. It's essentially a specialized predicate function- a -> Validation e (). The verify function simply combines these 2 potentially generalizable contravariant functors, into a very specialized usecase.

For a version of Valida that implements Profunctor and hence uses this generalized contravariance - check out valida!

Comparison and Motivation

The concept of the Validation data type used in this package isn't new. It's also used in the following packages-

Valida aims to be a minimal in terms of dependencies, but batteries included in terms of API. It borrows many philosophies from Data.Validation (from validation) and Validation (from validation-selective), and aims to provide a convenient, minimal way to model the common usecases of them.

The verify function, combined with the ValidationRule combinators, and the parsec-esque Validator aims to assist in easily modeling typical validation usecases without too much boilerplate. The core idea, really, is contravariance - the typical usecases, especially when validating product types (the most common target of validation), simply showcases contravariant functors.

In essence, the validation style itself, is designed to look like forma. Though the actual types, and core concepts are significantly different.