Note about semantics of (!!) is not specific enough, if it's correct
The definition of (!!)
in GHC.List
, copied below, has a vague note about semantics for the Prelude and HBC versions being different. I can't see the difference myself. If there really is a difference, the details should be in the comment.
-- | List index (subscript) operator, starting from 0.
-- It is an instance of the more general 'Data.List.genericIndex',
-- which takes an index of any integral type.
(!!) :: [a] -> Int -> a
#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
-- The semantics is not quite the same for error conditions
-- in the more efficient version.
--
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
Trac metadata
Trac field | Value |
---|---|
Version | 7.9 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | libraries/base |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | ekmett, hvr |
Operating system | |
Architecture |