Safe Haskell | None |
---|---|

Language | Haskell2010 |

- class Compactable f where
- fforMaybe :: (Compactable f, Functor f) => f a -> (a -> Maybe b) -> f b
- fmapMaybeM :: (Compactable f, Monad f) => (a -> MaybeT f b) -> f a -> f b
- fforMaybeM :: (Compactable f, Monad f) => f a -> (a -> MaybeT f b) -> f b
- applyMaybeM :: (Compactable f, Monad f) => f (a -> MaybeT f b) -> f a -> f b
- bindMaybeM :: (Compactable f, Monad f) => f a -> (a -> f (MaybeT f b)) -> f b
- traverseMaybeM :: (Monad m, Compactable t, Traversable t) => (a -> MaybeT m b) -> t a -> m (t b)

# Documentation

class Compactable f where Source #

This is a generalization of catMaybes as a new function compact. Compact has relations with Functor, Applicative, Monad, Alternative, and Traversable. In that we can use these class to provide the ability to operate on a data type by throwing away intermediate Nothings. This is useful for representing striping out values or failure.

To be compactable alone, no laws must be satisfied other than the type signature.

If the data type is also a Functor the following should hold:

*Kleisli composition*fmapMaybe (l <=< r) = fmapMaybe l . fmapMaybe r

*Functor identity 1*compact . fmap Just = id

*Functor identity 2*fmapMaybe Just = id

*Functor relation*compact = fmapMaybe id

According to Kmett, (Compactable f, Functor f) is a functor from the
kleisli category of Maybe to the category of haskell data types.
`Kleisli Maybe -> Hask`

.

If the data type is also Applicative the following should hold:

*Applicative left identity*compact . (pure Just *) = id

*Applicative right identity*applyMaybe (pure Just) = id

*Applicative relation*compact = applyMaybe (pure id)

If the data type is also a Monad the following should hold:

*Monad left identity*flip bindMaybe (return . Just) = id

*Monad right identity*compact . (return . Just =<<) = id

*Monad relation*compact = flip bindMaybe return

If the data type is also Alternative the following should hold:

*Alternative identity*compact empty = empty

*Alternative annihilation*compact (const Nothing <$> xs) = empty

If the data type is also Traversable the following should hold:

*Traversable Applicative relation*traverseMaybe (pure . Just) = pure

*Traversable composition*Compose . fmap (traverseMaybe f) . traverseMaybe g = traverseMaybe (Compose . fmap (traverseMaybe f) . g)

*Traversable Functor relation*traverse f = traverseMaybe (fmap Just . f)

*Traversable naturality*t . traverseMaybe f = traverseMaybe (t . f)

If you know of more useful laws, or have better names for the ones above (especially those marked "name me"). Please let me know.

compact :: f (Maybe a) -> f a Source #

compact :: (Monad f, Alternative f) => f (Maybe a) -> f a Source #

fmapMaybe :: Functor f => (a -> Maybe b) -> f a -> f b Source #

applyMaybe :: Applicative f => f (a -> Maybe b) -> f a -> f b Source #

bindMaybe :: Monad f => f a -> (a -> f (Maybe b)) -> f b Source #

traverseMaybe :: (Applicative g, Traversable f) => (a -> g (Maybe b)) -> f a -> g (f b) Source #

Compactable [] Source # | |

Compactable Maybe Source # | |

Compactable IO Source # | |

Compactable Option Source # | |

Compactable STM Source # | |

Compactable ReadPrec Source # | |

Compactable ReadP Source # | |

Compactable IntMap Source # | |

Compactable Seq Source # | |

Compactable Vector Source # | |

Monoid m => Compactable (Either m) Source # | |

Compactable (Proxy *) Source # | |

Compactable (Map k) Source # | |

Compactable (Const * r) Source # | |

(Compactable f, Compactable g) => Compactable (Product * f g) Source # | |

(Functor f, Functor g, Compactable g) => Compactable (Compose * * f g) Source # | |

fmapMaybeM :: (Compactable f, Monad f) => (a -> MaybeT f b) -> f a -> f b Source #

fforMaybeM :: (Compactable f, Monad f) => f a -> (a -> MaybeT f b) -> f b Source #

applyMaybeM :: (Compactable f, Monad f) => f (a -> MaybeT f b) -> f a -> f b Source #

bindMaybeM :: (Compactable f, Monad f) => f a -> (a -> f (MaybeT f b)) -> f b Source #

traverseMaybeM :: (Monad m, Compactable t, Traversable t) => (a -> MaybeT m b) -> t a -> m (t b) Source #