Monad, monad transformer, and typeclass representing choices.

The Monad Choice library contains monads, monad transformers, and a typeclass representing a sequence of choices of objects of arbitrary types where future choices can depend on previous ones.

The monad-choice package

This package provides a multipurpose monad transformer (and associated monad and class), to represent the idea of a result depending on the outcomes of a series of choices.

Example

The following is a simple use of MonadChoice for creating the name of a berry.

{-# Language FlexibleContexts #-}

berry :: MonadChoice NonEmpty m => m String
berry = do
  berryColor  <- choose $ "red"   :| ["blue", "orange", "yellow", "black"]
  berryFlavor <- choose $ "sweet" :| ["sour", "bitter"]
  (++ "berry") <$> choose ( berryColor :| [berryFlavor, berryColor ++ "-" ++ berryFlavor] )

This can be used with MonadRandom to create a random berry, used with Gen to create berries for unit tests, or with user input as the structure for a menu where the user selects a berry.