keksnicoh / parse-validate

toy haskell project to discover two data types

parse-validate

this is just a toy project. in a recent scala project, we had introduced the types

trait Parse[T] {
  def parse(s: String): Either[String, T]
  ...
}

trait Validate[T] {
  def validate(s: T): Either[String, T]
}

in order to combine simple parsers / validators which then can be used to derive json reads, path bindable, etc.

for exmaple, Parse[T] implies Reads[T]:

def reads[T: Parse]: Reads[T] = Reads[T] {
  case JsString(s) => implicitly[Parse[T]].parse(s).fold(JsError(_), JsSuccess(_))
  case _           => JsError("expected.string")
}

it is usefull when data is read via different channels to avoid duplication in the validation implementation. Turns out that the types have instances for Functor, Semigroup etc. For example, Validate[T] forms a semigroup, so validators can be combined associatively

someValidator = minLength 5 <> maxLength 10

i was interested in how the latter stuff would look like in haskell. the purpose of this repository is to practice haskell.

Installation

run

stack ghci

to run tests

stack test

Examples

Validate Contravariant Functor

stack ghci

import Data.Functor.Contravariant

data Derp = Derp { age :: Int, name :: String, flag :: Bool } deriving (Show)
:{
derpValidator =
  contramap age (ordMin 3 <> ordMax 1337)
    <> contramap name (maxLength 32)
    <> contramap flag (fromPredicate id)
:}
validate derpValidator (Derp 4 "peter schmidt" True)
-- Right (Derp {age = 4, name = "peter schmidt", flag = True})
validate derpValidator (Derp 1 "peter schmidt" True)
-- Left "validate.min"
validate derpValidator (Derp 3 "peter schmidt" False)
-- Left "validate.predicate"