How do I check the optimisation level of the built-in splitAt? Is something other than -O2 giving me my speed increase?

Simon Marlow marlowsd at
Thu May 15 05:31:32 EDT 2008

Richard Kelsall wrote:
> Hello Glasgow-Haskell Users,
> It was suggested to me in this thread in Haskell-Cafe
> which was a subsidiary of a previous thread
> that there might be some reason other than the -O2 optimisation level
> I applied to my version of splitAt that was making my program run about
> 30% faster than when I used the built-in splitAt.
> Can somebody tell me how to check what the -O level is for the built-in
> splitAt? Or alternatively tell me what the optimisation level is for
> the libraries.
> (Sorry I'm not sure of the right terminology for these built-in /
> library / Prelude things.)

It's hard to tell what optimisation level your libraries were compiled 
with.  The default setting is -O, but when building binary distributions we 
usually set it explicitly to -O2.  If you got your binary from another 
source, they might have only used -O.


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