Re: [Haskell-beginners] Can someone help to unnest this "do" cascade

8 Feb 2013

      On Wed, 06 Feb 2013 22:31:05 +0100, Martin Drautzburg
 wrote:
...
Can
someone please walk me through it and possibly show ways to avoid the  
massive
nesting.
dtz = do
    SndSeq.withDefault SndSeq.Block $ \h -> do
      Client.setName (h :: SndSeq.T SndSeq.DuplexMode) "Haskell-Melody"
      Port.withSimple h "out"
        (Port.caps [Port.capRead, Port.capSubsRead, Port.capWrite])
        (Port.types [Port.typeMidiGeneric, Port.typeApplication]) $ \p  
-> do
            Queue.with h $ \q -> do
              c <- Client.getId h
              let me = Addr.Cons c p
              conn <- parseDestArgs h me ["20:0"]
              Queue.control h q Event.QueueStart Nothing
              Queue.control h q (Event.QueueTempo (Event.Tempo  
10000000)) Nothing
              return ()
I like to divide large functions into several smaller ones:

dtz =
   SndSeq.withDefault SndSeq.Block f1
     where
       f1 h =
         do
           Client.setName (h :: SndSeq.T SndSeq.DuplexMode) "Haskell-Melody"
           Port.withSimple h "out"
             (Port.caps [Port.capRead, Port.capSubsRead, Port.capWrite])
             (Port.types [Port.typeMidiGeneric, Port.typeApplication]) (f2  
h)
       f2 h p = Queue.with h (f3 h p)
       f3 h p q =
         do
           c <- Client.getId h
           let me = Addr.Cons c p
           conn <- parseDestArgs h me ["20:0"]
           Queue.control h q Event.QueueStart Nothing
           Queue.control h q (Event.QueueTempo (Event.Tempo 10000000))  
Nothing
           return ()

f1, f2 and f3 might be replaced with more meaningful names. The "return
()" at the end can be removed; such things can be found with hlint[0].

Regards,
Henk-Jan van Tuyl

[0] http://hackage.haskell.org/package/hlint

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