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

[Ross Paterson]

On Tue, Jan 13, 2009 at 07:44:17PM -0500, Dan Doel wrote:
> On Tuesday 13 January 2009 7:27:10 pm Luke Palmer wrote:
> > 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.

But the only reason you want this monad optimized is so that you can
use it in do-notation.  Otherwise you'd just use Builder directly.