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Validation combinators for RxJs in PureScript
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README.md

Applicative style form validation with PureScript and RxJs

Module documentation

This package introduces two datatypes for working with form validation. Validator is for validating a single form field. It is a newtype wrapper around a function from the input value to the validation result Observable:

newtype Validator eff a b = Validator (a -> (Eff eff (Observable (Result b))))

This type is an instance of Semigroup and Applicative, so you can combine several validators to create new ones:

zipCode :: forall eff a. Validator eff String String
zipCode = minLength 5 *> maxLength 10 *> onlyNumbers

All of the failures will be concatted to the Result, if all of the validators succeed, the Result will hold the input value. Result is

type Result = Data.Validation.V [String] 

The failures are plain strings for now.

Validator is also a Category, so you can use composition. Improving from the previous one:

zipCode = required >>> (minLength 5 *> maxLength 10 *> onlyNumbers)

This would first check that the input is non-empty, and only then proceed to the other validators. There's also aliases for (>>>) and (<<<) with lower operator precedence:

zipCode = required &> minLength 5 *> maxLength 10 *> onlyNumbers

These validators didn't do any effects so why is there Eff in the type? Well, Ajax for one. You might also want to do other DOM effects in the validators, such as show progress. I'd like to replace the Eff and Observable with a single type variable (turning it into something that resembles a Kleisli arrow) but so far haven't been able to do so. This package should in the best case work as well with promises or continuations. Any suggestions on this is more than welcome.

You can run the validator with an Observable of input values and get an Observable of results:

main = do
  zipCodeV <- runValidator zipCode zipCodeInputs

Combining validation results

You may want to eventually combine all the field validators and do something when all of them are valid, like actually send the data to the server. For this there is a newtype wrapper around Observable (Result a):

newtype Validation a = Validation (Observable (Result a))

runValidator mentioned above returns these wrapped observables. Validate the whole form and do something with the result like so:

type Address = String
type ZipCode = String

data Address = Address Street ZipCode

main = do
  streetV <- runValidator street streetInputs
  zipCodeV <- runValidator zipCode zipCodeInputs
  subscribeValidation (Address <$> streetV <*> zipCodeV) doSomethingWithAddress
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