On Thursday 10 November 2011, 00:35:07, Artyom Kazak wrote:
Hello!
The (!) operator is short and nice. Unfortunately, when doing heavy computing, we have to use "unsafeAt" instead. It looks ugly and it is ugly, also.
Some compilers for imperative languages like Free Pascal have an option to turn on/off bounds checking for arrays. Wouldn't it be nice to have such option in GHC? Is it possible?
There is a problem: Haskell has a lot of array libraries. The only solution I see is a new FLAG pragma:
(!) :: Array i a -> i -> a --definition
{-# FLAG "boundsCheck" (!) = unsafeAt #-}
There's a problem here, unsafeAt uses an Int index into the array, while (!) uses the declared index type. Even skipping the bounds check, you'd still have to calculate the Int index for the replacement of (!).
It is similar to RULES pragma, but only fires when flag is set. To set the flag you need to complile with option -flags="boundsCheck". Also, the mantainers of vector library, bytestring library, repa library and so on will have to include such pragmas in their code.
I don't know about C++ preprocessor, though. Maybe this is already solvable with #define's...
#ifdef OMIT_BOUNDS_CHECK {-# RULES "ArrayIndex" arr ! i = unsafeAt arr (unsafeIndex (bounds arr) i) #-} #endif
Anyway, I have to say it once again: unsafeAt is ugly and Haskell is beautiful. Why high-performance code should be ugly?
(?) = unsafeAt