[Haskell-cafe] Re: Monads aren't evil? I think they are.

Dan Doel dan.doel at gmail.com
Tue Jan 13 19:44:17 EST 2009

On Tuesday 13 January 2009 7:27:10 pm Luke Palmer wrote:
> > When GHC starts optimizing (Writer Builder) as well as it optimizes PutM,
> > then
> > that will be a cogent argument. Until then, one might argue that it
> > misses "the whole point of Put".
> Well it can still serve as an optimization over bytestrings using whatever
> trickery it uses (I am assuming here -- I am not familiar with its
> trickery), the same way DList is an optimization over List.  It's just that
> its  monadiness is superfluous.
> Surely PutM and Writer Put have almost the same performance?!  (I am
> worried if not -- if not, can you give an indication why?)

The underlying monoid is Builder. The point of PutM is to be a version of 
Writer that's specialized to the Builder monoid for maximum performance. It 
looks like:

  data PairS a = PairS a {-# UNPACK #-} !Builder

  newtype PutM a = Put { unPut :: PairS a }

I'm not sure why it's split up like that. Anyhow, the strict, unpacked Builder 
gets optimized better than Writer Builder. Even if you change Writer to:

  data Writer w a = Writer a !w

it still won't match up, because polymorphic components don't get unpacked and 
such. That's, for instance, why Data.Sequence uses a specialized version of 
the finger tree type, instead of using the general version in Data.FingerTree.

Only exposing Put as a monoid is kind of redundant. You might as well work 
straight with Builder.

-- Dan

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