Could iteratee depend on mtl instead of transformers?
dan.doel at gmail.com
Tue Nov 17 22:27:29 EST 2009
On Tuesday 17 November 2009 9:56:21 pm Antoine Latter wrote:
> Every time this comes up I grow paranoid that there is some sort of
> general overhead to evaluating an expression of type "StateT s
> Identity a" vs "State s a".
> So I finally benchmarked it using the example code listed in
> Control.Monad.State, and found no difference whatsoever.
> Here's the source:
> I guess parsec is the odd one out, where there is a difference. Or I
> was measuring something else back when I had parsec benchmarks set up.
At least in the State case, everything involved is a newtype, so:
StateT s Identity a
should be represented identically to:
s -> Identity (a, s) = s -> (a, s)
Which is exactly what State s a is. From there it's just up to the compiler to
eliminate no-ops like:
return :: a -> a -- return :: a -> Identity a
return a = a
which shouldn't be too arduous. Building up stacks of novel monads can
introduce additional indirection over flattening them into a direct
implementation (like StateT s (WriterT w ...) versus RWST r s w ...), but
thankfully, Identity shouldn't introduce such overhead.
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