Documentation

Finally for the Error class. Errors in the finally part take precedence over prior errors.

Bracket without failure. Typically used to preserve state.

Lazy monadic disjunction.

The for version of mapMaybeMM.

Monadic if-then-else.

Like guard, but raise given error when condition fails.

Try a computation, return Nothing if an Error occurs.

Output a single value.

A monadic version of mapMaybe :: (a -> Maybe b) -> [a] -> [b].

A monadic version of dropWhile :: (a -> Bool) -> [a] -> [a].

Binary bind.

Monadic guard.

ifNotM mc = ifM (not $ mc)

Lazy monadic conjunction.

Lazy monadic disjunction with Either truth values. Returns the last error message if all fail.

Lazy monadic disjunction with accumulation of errors in a monoid. Errors are discarded if we succeed.

Generalized version of traverse_ :: Applicative m => (a -> m ()) -> [a] -> m () Executes effects and collects results in left-to-right order. Works best with left-associative monoids.

Note that there is an alternative

mapM' f t = foldr mappend mempty $ mapM f t

that collects results in right-to-left order (effects still left-to-right). It might be preferable for right associative monoids.

Generalized version of for_ :: Applicative m => [a] -> (a -> m ()) -> m ()

A version of mapMaybeM with a computation for the input list.

The for version of mapMaybeM.

A monadic version of dropWhileEnd :: (a -> Bool) -> [a] -> m [a]. Effects happen starting at the end of the list until p becomes false.

A `monadic' version of @partition :: (a -> Bool) -> [a] -> ([a],[a])

Translates Maybe to MonadPlus.

Generalises the catMaybes function from lists to an arbitrary MonadPlus.

Branch over elements of a monadic Foldable data structure.

Run a command, catch the exception and return it.

Restore state after computation.

Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

The reverse of when.

Monads that also support choice and failure.

An infix synonym for fmap.

The name of this operator is an allusion to $. Note the similarities between their types:

 ($)  ::              (a -> b) ->   a ->   b
(<$>) :: Functor f => (a -> b) -> f a -> f b

Whereas $ is function application, <$> is function application lifted over a Functor.

Examples

>>> show <$> Nothing
Nothing
>>> show <$> Just 3
Just "3"

>>> show <$> Left 17
Left 17
>>> show <$> Right 17
Right "17"

>>> (*2) <$> [1,2,3]
[2,4,6]

>>> even <$> (2,2)
(2,True)

Sequential application.

A few functors support an implementation of <*> that is more efficient than the default one.

Example

>>> data MyState = MyState {arg1 :: Foo, arg2 :: Bar, arg3 :: Baz}

>>> produceFoo :: Applicative f => f Foo

>>> produceBar :: Applicative f => f Bar
>>> produceBaz :: Applicative f => f Baz

>>> mkState :: Applicative f => f MyState
>>> mkState = MyState <$> produceFoo <*> produceBar <*> produceBaz

Replace all locations in the input with the same value. The default definition is fmap . const, but this may be overridden with a more efficient version.

Examples

>>> 'a' <$ Just 2
Just 'a'
>>> 'a' <$ Nothing
Nothing