[Haskell-cafe] Dynamic curiosity

[Gökhan San]

Hello All,

I've recently modified my existing project to support dynamic data 
(using 'Data.Dynamic'), and despite my earlier benchmarks on 'Dynamic', the 
program slowed down by a factor of 10. Profiler says that 'fromDynamic' is 
consuming 60% of the CPU time (with -O2).

Using the below function instead of 'fromDynamic' seems to make coercion 20 
times faster (thereby solving my problem):

> {-# OPTIONS -fglasgow-exts #-}

> import GHC.Prim

> data Dynamic' = Dynamic' TypeRep Any

> fromDynamic'   :: forall a. Typeable a => Dynamic -> Maybe a
> fromDynamic' d =
>     let Dynamic' t v = unsafeCoerce# d
>     in if t == typeOf (undefined :: a) 
>        then Just (unsafeCoerce# v) 
>        else Nothing  

I was able to reproduce the same result from scratch.

The only difference with the standard implementation (I put it below for 
reference), other than the first ugly unsafeCoerce#, seems to be existential 
quantification (i.e. When I replace "if t == typeOf (undefined :: a) ..." 
with the "case ..." below, performance goes down again.).

> fromDynamic :: Typeable a => Dynamic -> Maybe a
> fromDynamic (Dynamic t v) =
>     case unsafeCoerce v of 
>     r | t == typeOf r -> Just r
>       | otherwise     -> Nothing

So what is causing the improvement, or am I missing something?

Thanks.

-- Gokhan