[Haskell-cafe] Long running Haskell program

David Menendez dave at zednenem.com
Wed Nov 11 13:29:07 EST 2009

On Wed, Nov 11, 2009 at 1:09 PM, Matthew Pocock
<matthew.pocock at ncl.ac.uk> wrote:
> Is there a state monad that is strict on the state but lazy on the
> computation? Of course, strictness in the state will force a portion of the
> computation to be run, but there may be significant portions of it which are
> not run. Would there be a way to write a state monad such that it is
> entirely lazy, but then to wrap either the computation or the state in an
> 'eager' strategy datatype which takes care of this in a more flexible
> manner?

I think replacing "put s" with "put $! s" should guarantee that the
state is evaluated.

If you're using get and put in many place in the code, you could try
something along these lines:

newtype SStateT s m a = S { unS :: StateT s m a } deriving (Monad, etc.)

instance (Monad m) => MonadState s (SStateT s m) where
    get = S get
    put s = S (put $! s)

Dave Menendez <dave at zednenem.com>

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