mapM

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see mapM_.

Examples

mapM is literally a traverse with a type signature restricted to Monad. Its implementation may be more efficient due to additional power of Monad.

mapM f is equivalent to sequence . map f.

Apply a monadic transformation to all values in a stream. If you do not need the transformed values, and instead just want the monadic side-effects of running the action, see mapM_. Subject to fusion

Apply a monadic transformation to all values in a stream. If you do not need the transformed values, and instead just want the monadic side-effects of running the action, see mapM_. Subject to fusion Since 0.3.0

Apply a monadic operation to each element of a Stream, lazily

Apply a monadic function to all values flowing downstream

mapM return = cat

mapM (f >=> g) = mapM f >-> mapM g

Maps an impure function over an InputStream. mapM f s passes all output from s through the IO action f. Satisfies the following laws:

Streams.mapM (f >=> g) === Streams.mapM f >=> Streams.mapM g
Streams.mapM return === Streams.makeInputStream . Streams.read

Replace each element of a stream with the result of a monadic action

>>> S.print $ S.mapM readIORef $ S.chain (\ior -> modifyIORef ior (*100)) $ S.mapM newIORef $ each [1..6]
100
200
300
400
500
600

See also chain for a variant of this which ignores the return value of the function and just uses the side effects.

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see mapM_.

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results

A map in monad space. Same as sequence . map See also rigidMapM

mapM f = sequence . map f

Apply a monadic function to each element of the stream and replace it with the output of the resulting action.

>>> drain $ Stream.mapM putStr $ Stream.fromList ["a", "b", "c"]
abc

>>> :{
drain $ Stream.replicateM 10 (return 1)
& (fromSerial . Stream.mapM (x -> threadDelay 1000000 >> print x))
:}
1
...
1

> drain $ Stream.replicateM 10 (return 1)
& (fromAsync . Stream.mapM (x -> threadDelay 1000000 >> print x))

Concurrent (do not use with fromParallel on infinite streams)

A mapM equivalent.

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. for a version that ignores the results see mapM_.

Like mapM, but applying the function to the individual list items in parallel.

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results.

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results.

O(n) Apply the monadic action to all elements of the vector, yielding a vector of results.