GHC.Arr

[Simon Peyton-Jones]

Right. I'll make some specialised instances in GHC.Arr.  And add comments to explain the issue

S

| -----Original Message-----
| From: Simon Marlow [mailto:marlowsd at gmail.com]
| Sent: 18 December 2009 09:31
| To: Duncan Coutts
| Cc: Simon Peyton-Jones; Simon Marlow; cvs-ghc at haskell.org
| Subject: Re: GHC.Arr
| 
| On 18/12/09 05:48, Duncan Coutts wrote:
| > On Thu, 2009-12-17 at 18:03 +0000, Simon Peyton-Jones wrote:
| >> Simon
| >>
| >> I'm puzzling over GHC.Arr.safeIndex
| >>
| >> It calls the overloaded method 'index' which does bound checks on the
| >> "semantic range", by checking that i is in the range (l,u).  But then
| >> safeIndex does *another* range check, on the resulting index value.
| >>
| >> Shouldn't it be an invariant that if index (l,u) i returns at all, it
| >> returns an in-range value?
| >>
| >> I suppose we can't enforce that.  But *by default* doing two range
| >> checks on every array access seems stupid.
| >
| > So far we've been through three iterations of this thing :-)
| >
| > The original version did one range check, the Ix one. Then people
| > complained that custom Ix instances were unsafe because they could index
| > outside the array.
| >
| > The next iteration just checked the resulting index value. Then people
| > complained because you could index outside the Ix range (eg on 2-d
| > arrays) without any error being raised.
| >
| > The current iteration now does both checks. Now people complain that
| > it's slow :-)
| >
| > So the challenge is to satisfy all these requirements simultaneously.
| 
| We could omit the Ix check for simple scalar types where the range check
| is sufficient.  For 2-d arrays and suchlike we could do the Ix check but
| omit the range check.  User-defined Ix instances would get both an Ix
| check and a range check unless they explicitly use the unsafe methods.
| 
| Cheers,
| 	Simon