#1046: Make array indexing immune to seg-faults
--------------------------------+-------------------------------------------
Reporter: simonpj | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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As Spencer Janssen points out (http://www.haskell.org/pipermail/libraries
/2006-December/006539.html), it's possible for a bogus instance of `Ix` to
cause a Haskell implementation to seg-fault, simply by returning an out-
of-range index. This is definitely a Bad Thing.
The only way to avoid this possibility is to make `(!)` perform a bounds
check ''after'' calling the `index` method of class `Ix`. GHC's current
implementation (in `GHC.Arr`) is
{{{
(!) :: Ix i => Array i e -> i -> e
arr@(Array l u _) ! i = unsafeAt arr (index (l,u) i)
}}}
Instead we could have
{{{
(!) :: Ix i => Array i e -> i -> e
arr@(Array l u _) ! i = safeAt arr (index (l,u) i)
}}}
where `safeAt` performs a bounds check. But that would ''two'' bounds
checks, one in `index` and one in `safeAt`. We could eliminate one by
using `unsafeIndex`, which is a (usually hidden) method of GHC's `Ix`
class definition. However, that might give rise to less-informative
messages when the bounds check fails.
To implement `safeAt`, we'd need a new primop:
{{{
arraySize :: Array# a -> Int
}}}
There would need to be corresponding stuff for `Data.Array.IArray` and
`Data.Array.MArray`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1046>
GHC <http://www.haskell.org/ghc/>
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