[Haskell-cafe] What pattern is this (Something.T -> IO a) in Sound.ALSA.Sequencer

[Martin Drautzburg]

Hello all,

this was previously posted on Haskell Beginners, but only partially answered.

In Sound.ALSA.Sequencer, there are a number of functions which together set up 
a midi environement (client, port, queue). They all have a type, where the 
last argument has a type like:

(something.T -> IO a)

i.e.

*Main> :t SndSeq.withDefault
SndSeq.withDefault
  :: SndSeq.OpenMode mode =>
     SndSeq.BlockMode -> (SndSeq.T mode -> IO a) -> IO a

*Main> :t Port.withSimple
Port.withSimple
  :: SndSeq.T mode
     -> String -> Port.Cap -> Port.Type -> (Port.T -> IO a) -> IO a

*Main> :t Queue.with
Queue.with :: SndSeq.T mode -> (Queue.T -> IO a) -> IO a

There is example code, where a full setup is created by a number of nested 
"do" blocks. The repeating pattern there is:

something1 $ \x -> do
        something2 $ \y -> do
                something3 $ \z -> do


What is this all about? I particularly would like to understand, when this 
parttern is needed and what determines the the number of nested "do" blocks. 

-- 
Martin