[Haskell-cafe] mdo with multiple values

Joachim Breitner mail at joachim-breitner.de
Wed Jan 15 23:24:05 UTC 2014

Dear John,

the background is a binary format assembler, so you can think of the
monad as the Put monad, only sufficiently lazy to admit a useful
MonadFix instance. Then one can do nice things like

          putWord32 offset
          putBS someMoreHeaderData
          offset <- getCurrentOffset
          putBS byteString1

where I conceptually use the offset before it is known.
So what if I want to put two offsets, followed by two bytestrings? Easy:

        putWord32 offset1
        putWord32 offset2
        putBS someMoreHeaderData
        offset1 <- getCurrentOffset
        putBS byteString1
        offset2 <- getCurrentOffset
        putBS byteString2
Now I try to generalize that to a list of Bytestrings, and just from the
looks of it, this is what you want to do:

        mapM_ putWord32 offsets
        putBS someMoreHeaderData
        offsets <- forM byteStrings $ \b -> 
           offset <- getCurrentOffset
           putBS b
           return offset

but that will <<loop>>.

Am Mittwoch, den 15.01.2014, 13:10 -0800 schrieb John Lato:
> Hello Joachim, 
> I don't really understand what you're doing here. There's the obvious
>   mapM_ (act2 >=> act1)
> But presumably act1 performs some monadic action that doesn't depend
> on its input, and you need that to be performed before act2?

Exactly, (act2 >=> act1) would write out the data in the wrong order.

> To me, it feels like there's some sort of implicit coupling between
> act1 and act2, and you would be better off extracting that, perhaps by
> changing act1 to have the type act1 :: M T -> () .
> If that's not possible, your approach seems pretty simple to me. 

I don’t think its, possible with that signature, no...

(It were if I were to interleave the calls to act1 and act2, instead of
requiring first all act1 and then all act2, but then it would be trivial


Joachim Breitner
  e-Mail: mail at joachim-breitner.de
  Homepage: http://www.joachim-breitner.de
  Jabber-ID: nomeata at joachim-breitner.de

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