I'm generally +1 on this concept, but I'd like to see some evidence that
there is no measurable performance impact on nofib and perhaps also other
benchmarks, particularly for very short ephemeral lists. Note that saving
the index requires actually putting it somewhere.
On Oct 16, 2014 2:21 AM, "Simon Hengel"
Hi, currently we have the following implementation for (!!):
#ifdef USE_REPORT_PRELUDE xs !! n | n < 0 = error "Prelude.!!: negative index" [] !! _ = error "Prelude.!!: index too large" (x:_) !! 0 = x (_:xs) !! n = xs !! (n-1) #else -- HBC version (stolen), then unboxified xs !! (I# n0) | isTrue# (n0 <# 0#) = error "Prelude.(!!): negative index\n" | otherwise = sub xs n0 where sub :: [a] -> Int# -> a sub [] _ = error "Prelude.(!!): index too large\n" sub (y:ys) n = if isTrue# (n ==# 0#) then y else sub ys (n -# 1#) #endif
I propose to change the error messages for the non-report version to include index and list length, something that is functionally equivalent to:
xs !! (I# n0) | isTrue# (n0 <# 0#) = indexError xs n0 | otherwise = sub xs n0 where
sub :: [a] -> Int# -> a sub [] _ = indexError xs n0 sub (y:ys) n = if isTrue# (n ==# 0#) then y else sub ys (n -# 1#) indexError :: [a] -> Int# -> b indexError xs (I# -> n) | n < 0 = error ("Prelude.(!!): negative index " ++ show n) | otherwise = error ("Prelude.(!!): index " ++ show n ++ " too large for list of length " ++ show (length xs))
Some usage examples:
*Main> [1, 2, 3] !! (-1) *** Exception: Prelude.(!!): negative index -1 *Main> [1, 2, 3] !! 3 *** Exception: Prelude.(!!): index 3 too large for list of length 3
This will require some refactoring, i.e. we need to move itos from GHC.Show to e.g. GHC.Base.
Discussion period: 2 weeks
Cheers, Simon _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries