Example demonstrating Alternative

alternative.hs

{-# LANGUAGE NoImplicitPrelude #-}

module AlternativeExample where

import Control.Applicative
import Data.Maybe
import Data.Text           hiding (empty)
import GHC.Show            (Show)

type Login = Text
type Email = Text
type Bio = Text

data User = MkUser Login Email Bio deriving (Show)

-- In a real world application this would be validating if the @Login@ is valid (no curse words, or whatever that means)
getFormLogin :: Applicative f => f Login
getFormLogin = pure "curlyfries"

-- In a real world application this would be validating the format of the email address given in the form
getFormEmail :: Applicative f => f Email
getFormEmail = pure "[email protected]"

-- In a real world application this would check for spam or something
getFormBio :: Applicative f => f Bio
getFormBio = pure "Here is the bio for this user."

-- Not sure having a default user makes sense in all systems, but it might.
defaultUser :: User
defaultUser = MkUser "admin" "[email protected]" "some bio"

baseExpr :: Applicative f => f Bio -> f User
baseExpr bio = MkUser <$> getFormLogin <*> getFormEmail <*> bio

-- Produces
-- >>> expr0
-- Just (MkUser "curlyfries" "[email protected]" "Here is the bio for this user.")
expr0 :: Maybe User
expr0 = baseExpr getFormBio <|> pure defaultUser

-- Produces
-- >>> expr1
-- Just (MkUser "admin" "[email protected]" "some bio")
expr1 :: Maybe User
expr1 = baseExpr Nothing <|> Just defaultUser

-- Produces
-- >>> expr2
-- Nothing
expr2 :: Maybe User
expr2 = baseExpr Nothing <|> Nothing

-- TL;DR the applicative interface allows you to build up expressions that inject fully formed values into a context and 
-- the alternative interface gives you the ability to provide alternative/"otherwise" paths in expressions such that
-- more logic can be composed around it in these contexts or via these or related interfaces.