On Mon, Jun 04, 2007 at 09:43:32AM +0100, Alistair Bayley wrote:
(The docs tell me that using GHC.Exts is the "approved" way of accessing GHC-specific extensions, but all of the useful stuff seems to be in GHC.Prim.)
All of the useful stuff *is* exported from GHC.Exts, it even says so in the haddock: Synopsis ... module GHC.Prim That is, GHC.Exts exports everything GHC.Prim does. Standard H98 re-export syntax. Besides, user code can't import GHC.Prim at all in GHC HEADs newer than a couple months (arguably a bug, but it only breaks bad code, so...)
Some things I've noticed in the simplifier output: - the shiftL call hasn't unboxed or inlined into a call to uncheckedShiftL#, which I would prefer. Would this be possible if we added unchecked versions of the shiftL/R functions to Data.Bits? - Ptrs don't get unboxed. Why is this? Some IO monad thing?
fromUTF8Ptr unboxes fine for me with HEAD and 6.6.1.
- the chr function tests that its Int argument is less than 1114111, before constructing the Char. It'd be nice to avoid this test.
You want unsafeChr from the (undocumented) GHC.Base module. http://darcs.haskell.org/ghc-6.6/packages/base/GHC/Base.lhs for reference (but don't copy the file, it's already an importable module).
- why does this code:
| x <= 0xF7 = remaining 3 (bAND x 0x07) xs | otherwise = err x
turn into this i.e. the <= turns into two identical case-branches, using eqword# and ltword#, rather than one case-branch using leword# ?
case GHC.Prim.eqWord# a11_a2PJ __word 247 of wild25_X2SU { GHC.Base.False -> case GHC.Prim.ltWord# a11_a2PJ __word 247 of wild6_Xcw { GHC.Base.False -> <error call> GHC.Base.True -> $wremaining_r3dD 3 (__scc {fromUTF8 main:Foreign.C.UTF8 !} GHC.Base.I# (GHC.Prim.word2Int# (GHC.Prim.and# a11_a2PJ __word 7))) xs_aVm }; GHC.Base.True -> $wremaining_r3dD 3 (__scc {fromUTF8 main:Foreign.C.UTF8 !} GHC.Base.I# (GHC.Prim.word2Int# (GHC.Prim.and# a11_a2PJ __word 7))) xs_aVm };
ISTR seeing a bug report about this a while back, we know it's dumb. You could probably use x < 0xF8 instead.
BTW, what's the difference between the indexXxxxOffAddr# and readXxxxOffAddr# functions in GHC.Prim? AFAICT they are equivalent, except that the read* functions take an extra State# s parameter. Presumably this is to thread the IO monad's RealWorld value through, to create some sort of data dependency between the functions (and so to ensure ordered evaluation?)
Exactly. readFoo won't be reordered, indexFoo will - which matters when doing reads and writes at addresses that might alias. Stefan