lift package:rebase

Lift a computation from the argument monad to the constructed monad.

Lift a function to actions. Equivalent to Functor's fmap but implemented using only Applicative's methods: liftA f a = pure f <*> a As such this function may be used to implement a Functor instance from an Applicative one.

Examples

Using the Applicative instance for Lists:

>>> liftA (+1) [1, 2]
[2,3]

Or the Applicative instance for Maybe

>>> liftA (+1) (Just 3)
Just 4

Lift a binary function to actions. Some functors support an implementation of liftA2 that is more efficient than the default one. In particular, if fmap is an expensive operation, it is likely better to use liftA2 than to fmap over the structure and then use <*>. This became a typeclass method in 4.10.0.0. Prior to that, it was a function defined in terms of <*> and fmap.

Example

>>> liftA2 (,) (Just 3) (Just 5)
Just (3,5)

Lift a ternary function to actions.

Lift a compare function through the type constructor. The function will usually be applied to a comparison function, but the more general type ensures that the implementation uses it to compare elements of the first container with elements of the second.

Lift compare functions through the type constructor. The function will usually be applied to comparison functions, but the more general type ensures that the implementation uses them to compare elements of the first container with elements of the second.

This is the divisible analogue of liftA. It gives a viable default definition for contramap in terms of the members of Divisible.

liftD f = divide ((,) () . f) conquer

Lifts an Either e into any MonadError e.

do { val <- liftEither =<< action1; action2 }

where action1 returns an Either to represent errors.

Lift an equality test through the type constructor. The function will usually be applied to an equality function, but the more general type ensures that the implementation uses it to compare elements of the first container with elements of the second.

Lift equality tests through the type constructor. The function will usually be applied to equality functions, but the more general type ensures that the implementation uses them to compare elements of the first container with elements of the second.

Lift a binary function into a comonad with zipping

Lift a ternary function into a comonad with zipping

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
state <- get
liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
Expected type: StateT s IO ()
Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO (). Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

liftLocal ask local yields a local function for ContT r m.

Promote a function to a monad.

Promote a function to a monad, scanning the monadic arguments from left to right. For example,

liftM2 (+) [0,1] [0,2] = [0,2,1,3]
liftM2 (+) (Just 1) Nothing = Nothing

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

readList function for an application of the type constructor based on readsPrec and readList functions for the argument type. The default implementation using standard list syntax is correct for most types.

readList function for an application of the type constructor based on readsPrec and readList functions for the argument types. The default implementation using standard list syntax is correct for most types.

A possible replacement definition for the liftReadList2 method. This is only needed for Read2 instances where liftReadListPrec2 isn't defined as liftReadListPrec2Default.

A possible replacement definition for the liftReadList method. This is only needed for Read1 instances where liftReadListPrec isn't defined as liftReadListPrecDefault.

readListPrec function for an application of the type constructor based on readPrec and readListPrec functions for the argument type. The default definition uses liftReadList. Instances that define liftReadPrec should also define liftReadListPrec as liftReadListPrecDefault.