Either.purs

module Data.Either where

import Prelude

import Control.Alt (class Alt, (<|>))
import Control.Extend (class Extend)
import Data.Bifoldable (class Bifoldable)
import Data.Bifunctor (class Bifunctor)
import Data.Bitraversable (class Bitraversable)
import Data.Eq (class Eq1)
import Data.Foldable (class Foldable)
import Data.FoldableWithIndex (class FoldableWithIndex)
import Data.Functor.Invariant (class Invariant, imapF)
import Data.FunctorWithIndex (class FunctorWithIndex)
import Data.Maybe (Maybe(..), maybe, maybe')
import Data.Ord (class Ord1)
import Data.Traversable (class Traversable)
import Data.TraversableWithIndex (class TraversableWithIndex)

-- | The `Either` type is used to represent a choice between two types of value.
-- |
-- | A common use case for `Either` is error handling, where `Left` is used to
-- | carry an error value and `Right` is used to carry a success value.
data Either a b = Left a | Right b

-- | The `Functor` instance allows functions to transform the contents of a
-- | `Right` with the `<$>` operator:
-- |
-- | ``` purescript
-- | f <$> Right x == Right (f x)
-- | ```
-- |
-- | `Left` values are untouched:
-- |
-- | ``` purescript
-- | f <$> Left y == Left y
-- | ```
derive instance functorEither :: Functor (Either a)

instance functorWithIndexEither :: FunctorWithIndex Unit (Either a) where
  mapWithIndex f = map $ f unit

instance invariantEither :: Invariant (Either a) where
  imap = imapF

instance bifunctorEither :: Bifunctor Either where
  bimap f _ (Left l) = Left (f l)
  bimap _ g (Right r) = Right (g r)

-- | The `Apply` instance allows functions contained within a `Right` to
-- | transform a value contained within a `Right` using the `(<*>)` operator:
-- |
-- | ``` purescript
-- | Right f <*> Right x == Right (f x)
-- | ```
-- |
-- | `Left` values are left untouched:
-- |
-- | ``` purescript
-- | Left f <*> Right x == Left x
-- | Right f <*> Left y == Left y
-- | ```
-- |
-- | Combining `Functor`'s `<$>` with `Apply`'s `<*>` can be used to transform a
-- | pure function to take `Either`-typed arguments so `f :: a -> b -> c`
-- | becomes `f :: Either l a -> Either l b -> Either l c`:
-- |
-- | ``` purescript
-- | f <$> Right x <*> Right y == Right (f x y)
-- | ```
-- |
-- | The `Left`-preserving behaviour of both operators means the result of
-- | an expression like the above but where any one of the values is `Left`
-- | means the whole result becomes `Left` also, taking the first `Left` value
-- | found:
-- |
-- | ``` purescript
-- | f <$> Left x <*> Right y == Left x
-- | f <$> Right x <*> Left y == Left y
-- | f <$> Left x <*> Left y == Left x
-- | ```
instance applyEither :: Apply (Either e) where
  apply (Left e) _ = Left e
  apply (Right f) r = f <$> r

-- | The `Applicative` instance enables lifting of values into `Either` with the
-- | `pure` function:
-- |
-- | ``` purescript
-- | pure x :: Either _ _ == Right x
-- | ```
-- |
-- | Combining `Functor`'s `<$>` with `Apply`'s `<*>` and `Applicative`'s
-- | `pure` can be used to pass a mixture of `Either` and non-`Either` typed
-- | values to a function that does not usually expect them, by using `pure`
-- | for any value that is not already `Either` typed:
-- |
-- | ``` purescript
-- | f <$> Right x <*> pure y == Right (f x y)
-- | ```
-- |
-- | Even though `pure = Right` it is recommended to use `pure` in situations
-- | like this as it allows the choice of `Applicative` to be changed later
-- | without having to go through and replace `Right` with a new constructor.
instance applicativeEither :: Applicative (Either e) where
  pure = Right

-- | The `Alt` instance allows for a choice to be made between two `Either`
-- | values with the `<|>` operator, where the first `Right` encountered
-- | is taken.
-- |
-- | ``` purescript
-- | Right x <|> Right y == Right x
-- | Left x <|> Right y == Right y
-- | Left x <|> Left y == Left y
-- | ```
instance altEither :: Alt (Either e) where
  alt (Left _) r = r
  alt l        _ = l

-- | The `Bind` instance allows sequencing of `Either` values and functions that
-- | return an `Either` by using the `>>=` operator:
-- |
-- | ``` purescript
-- | Left x >>= f = Left x
-- | Right x >>= f = f x
-- | ```
instance bindEither :: Bind (Either e) where
  bind = either (\e _ -> Left e) (\a f -> f a)

-- | The `Monad` instance guarantees that there are both `Applicative` and
-- | `Bind` instances for `Either`. This also enables the `do` syntactic sugar:
-- |
-- | ``` purescript
-- | do
-- |   x' <- x
-- |   y' <- y
-- |   pure (f x' y')
-- | ```
-- |
-- | Which is equivalent to:
-- |
-- | ``` purescript
-- | x >>= (\x' -> y >>= (\y' -> pure (f x' y')))
-- | ```
instance monadEither :: Monad (Either e)

-- | The `Extend` instance allows sequencing of `Either` values and functions
-- | that accept an `Either` and return a non-`Either` result using the
-- | `<<=` operator.
-- |
-- | ``` purescript
-- | f <<= Left x = Left x
-- | f <<= Right x = Right (f (Right x))
-- | ```
instance extendEither :: Extend (Either e) where
  extend _ (Left y)  = Left y
  extend f x         = Right (f x)

-- | The `Show` instance allows `Either` values to be rendered as a string with
-- | `show` whenever there is an `Show` instance for both type the `Either` can
-- | contain.
instance showEither :: (Show a, Show b) => Show (Either a b) where
  show (Left x) = "(Left " <> show x <> ")"
  show (Right y) = "(Right " <> show y <> ")"

-- | The `Eq` instance allows `Either` values to be checked for equality with
-- | `==` and inequality with `/=` whenever there is an `Eq` instance for both
-- | types the `Either` can contain.
derive instance eqEither :: (Eq a, Eq b) => Eq (Either a b)

derive instance eq1Either :: Eq a => Eq1 (Either a)

-- | The `Ord` instance allows `Either` values to be compared with
-- | `compare`, `>`, `>=`, `<` and `<=` whenever there is an `Ord` instance for
-- | both types the `Either` can contain.
-- |
-- | Any `Left` value is considered to be less than a `Right` value.
derive instance ordEither :: (Ord a, Ord b) => Ord (Either a b)

derive instance ord1Either :: Ord a => Ord1 (Either a)

instance boundedEither :: (Bounded a, Bounded b) => Bounded (Either a b) where
  top = Right top
  bottom = Left bottom

instance foldableEither :: Foldable (Either a) where
  foldr _ z (Left _)  = z
  foldr f z (Right x) = f x z
  foldl _ z (Left _)  = z
  foldl f z (Right x) = f z x
  foldMap f (Left _)  = mempty
  foldMap f (Right x) = f x

instance foldableWithIndexEither :: FoldableWithIndex Unit (Either a) where
  foldrWithIndex _ z (Left _)  = z
  foldrWithIndex f z (Right x) = f unit x z
  foldlWithIndex _ z (Left _)  = z
  foldlWithIndex f z (Right x) = f unit z x
  foldMapWithIndex f (Left _)  = mempty
  foldMapWithIndex f (Right x) = f unit x

instance bifoldableEither :: Bifoldable Either where
  bifoldr f _ z (Left a) = f a z
  bifoldr _ g z (Right b) = g b z
  bifoldl f _ z (Left a) = f z a
  bifoldl _ g z (Right b) = g z b
  bifoldMap f _ (Left a) = f a
  bifoldMap _ g (Right b) = g b

instance traversableEither :: Traversable (Either a) where
  traverse _ (Left x)  = pure (Left x)
  traverse f (Right x) = Right <$> f x
  sequence (Left x) = pure (Left x)
  sequence (Right x)  = Right <$> x

instance traversableWithIndexEither :: TraversableWithIndex Unit (Either a) where
  traverseWithIndex _ (Left x)  = pure (Left x)
  traverseWithIndex f (Right x) = Right <$> f unit x

instance bitraversableEither :: Bitraversable Either where
  bitraverse f _ (Left a) = Left <$> f a
  bitraverse _ g (Right b) = Right <$> g b
  bisequence (Left a) = Left <$> a
  bisequence (Right b) = Right <$> b

instance semigroupEither :: (Semigroup b) => Semigroup (Either a b) where
  append x y = append <$> x <*> y

-- | Takes two functions and an `Either` value, if the value is a `Left` the
-- | inner value is applied to the first function, if the value is a `Right`
-- | the inner value is applied to the second function.
-- |
-- | ``` purescript
-- | either f g (Left x) == f x
-- | either f g (Right y) == g y
-- | ```
either :: forall a b c. (a -> c) -> (b -> c) -> Either a b -> c
either f _ (Left a) = f a
either _ g (Right b) = g b

-- | Combine two alternatives.
choose :: forall m a b. Alt m => m a -> m b -> m (Either a b)
choose a b = Left <$> a <|> Right <$> b

-- | Returns `true` when the `Either` value was constructed with `Left`.
isLeft :: forall a b. Either a b -> Boolean
isLeft = either (const true) (const false)

-- | Returns `true` when the `Either` value was constructed with `Right`.
isRight :: forall a b. Either a b -> Boolean
isRight = either (const false) (const true)

-- | A partial function that extracts the value from the `Left` data constructor.
-- | Passing a `Right` to `fromLeft` will throw an error at runtime.
fromLeft :: forall a b. Partial => Either a b -> a
fromLeft (Left a) = a

-- | A partial function that extracts the value from the `Right` data constructor.
-- | Passing a `Left` to `fromRight` will throw an error at runtime.
fromRight :: forall a b. Partial => Either a b -> b
fromRight (Right a) = a

-- | Takes a default and a `Maybe` value, if the value is a `Just`, turn it into
-- | a `Right`, if the value is a `Nothing` use the provided default as a `Left`
-- |
-- | ```purescript
-- | note "default" Nothing = Left "default"
-- | note "default" (Just 1) = Right 1
-- | ```
note :: forall a b. a -> Maybe b -> Either a b
note a = maybe (Left a) Right

-- | Similar to `note`, but for use in cases where the default value may be
-- | expensive to compute.
-- |
-- | ```purescript
-- | note' (\_ -> "default") Nothing = Left "default"
-- | note' (\_ -> "default") (Just 1) = Right 1
-- | ```
note' :: forall a b. (Unit -> a) -> Maybe b -> Either a b
note' f = maybe' (Left <<< f) Right

-- | Turns an `Either` into a `Maybe`, by throwing eventual `Left` values away and converting
-- | them into `Nothing`. `Right` values get turned into `Just`s.
-- |
-- | ```purescript
-- | hush (Left "ParseError") = Nothing
-- | hush (Right 42) = Just 42
-- | ```
hush :: forall a b. Either a b -> Maybe b
hush = either (const Nothing) Just