Re: Proposal: Improve error messages for (!!) (include index and length of list)

You don't need an extra parameter for calculating the length, because len = n - idx So, xs !! (I# n) = go xs n where go [] idx = error $ "... Index " ++ show (I# n) ++ " too large for list of length" ++ show (I# (n -# idx)) go (x:_) 0# = x go (_:xs) idx = go xs (idx +# 1) By the way, do we still need all the manual unboxing with a modern Ghc? Twan On 2014-10-16 14:13, Thomas Schilling wrote:

Yes, you'd have to calculate the length on the fly. i.e., something like this (untested):

xs !! n | n < 0 = error "... negative index ..." xs !! (I# n) = go xs n 0# where go [] idx len = error $ "... Index " ++ show (I# (idx +# len) ++ " too large for list of length " ++ show (I# len) go (x:_) 0# _ = x go (_:xs) idx len = go xs (idx -# 0#) (len +# 1#)

On modern processors the extra addition and the extra parameter shouldn't hurt, though we'd need a benchmark to make sure, of course. You could also make the error message a bit less helpful and just return how far the index pointed past the end of the list.

On 16 October 2014 13:46, Herbert Valerio Riedel wrote:

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On 2014-10-16 at 08:20:55 +0200, Simon Hengel wrote:

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I propose to change the error messages for the non-report version to include index and list length, something that is functionally equivalent to:

While I'm very sympathetic to better error messages; doesn't the implementation you gave defer garbage-collecting the start of the list, by keeping the head of the list alive until either the desired index has been reached or end-of-list is detected?

e.g. consider something (silly) like ([1..] !! 10000000)

Cheers, hvr _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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