Control.FromSum

This Haskell module exports various "from" functions for Either and Maybe.

Monadic in return value

fromEitherM :: Applicative m => (e -> m a) -> Either e a -> m a

A monadic version of fromEither.

>>> fromEitherM (\s -> [length s]) $ Right 5
[5]
>>> fromEitherM (\s -> [length s]) $ Left ("foo" :: String)
[3]

fromMaybeM :: Applicative m => m a -> Maybe a -> m a

A monadic version of fromMaybe.

>>> fromMaybeM [] $ Just 5
[5]
>>> fromMaybeM [] Nothing
[]

Monadic in both return and sum-type value

fromEitherMM :: Monad m => (e -> m a) -> m (Either e a) -> m a

Similar to fromEitherM but Either argument is also a monadic value.

>>> fromEitherMM (\s -> [length s]) . pure $ Right 5
[5]
>>> fromEitherMM (\s -> [length s]) . pure $ Left ("foo" :: String)
[3]

• NOTE*: I don't particularly like the name of this function. If you have a suggestion for a better name, please submit a PR or issue.

fromMaybeMM :: Monad m => m a -> m (Maybe a) -> m a

Completely non-monadic functions

fromEither :: (e -> a) -> Either e a -> a

Similar to fromMaybe.

>>> fromEither show $ Left 5
"5"
>>> fromEither show $ Right "hello"
"hello"

fromMaybe :: a -> Maybe a -> a

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )
>>> fromMaybe 0 (readMaybe "5")
5
>>> fromMaybe 0 (readMaybe "")
0