[Haskell-beginners] Re: Code help requested

[Joe Van Dyk]

On Sat, Jan 16, 2010 at 5:30 AM, Brandon S. Allbery KF8NH
<allbery at ece.cmu.edu> wrote:
> On Jan 15, 2010, at 20:17 , Tim Perry wrote:
>>
>> binary_find list elem =
>>  do_search list elem 0 (length list -1)
>>  where
>>   do_search list elem low high
>>     | high < low = Nothing
>>     | midVal > elem = do_search list elem low (mid - 1)
>>     | midVal < elem = do_search list elem (mid + 1) high
>>     | otherwise = Just mid
>>     where
>>       midVal = list !! mid
>>       mid = low + (high - low) `div` 2
>
>
> Observation:  the first two parameters to do_search never change within an
> invocation of binary_find, and the corresponding arguments to binary_find
> are in scope; depending on the (lack of) cleverness of the compiler, you
> could see a speedup by not passing them around unnecessarily.  In addition,
> it's *conceptually* cleaner because passing them around explicitly suggests
> to someone reading the source that they *do* change when that isn't actually
> the case.

Good one, thanks.  The reason I had them in there originally is
because I was thinking it would be simpler if a function was passed in
all the inputs it was working with -- instead of referring to
variables defined outside its scope.

Joe