[Haskell-cafe] A few questions on primes generating.
sebastian.sylvan at gmail.com
Mon Aug 13 18:41:04 EDT 2007
On 13/08/07, Pekka Karjalainen <p3k at iki.fi> wrote:
> On 8/13/07, L.Guo <leaveye.guo at gmail.com> wrote:
> > Hi All:
> > I am reading http://www.haskell.org/haskellwiki/Prime_numbers
> > The code in sector "1 Bitwise prime sieve".
> > I have 3 questions about it.
> > 1) In function go, what does the number 46340 mean ? Is it sqrt(MAX_LONG) ?
> Yes, it appears so. In a 32 bit implementation I get:
> Prelude> sqrt $ fromIntegral (maxBound :: Int)
> > 2) We have this type definition :
> > pureSieve :: Int -> Int
> > Why there is no error (type mismatch) of this call in func main :
> > pureSieve 10000000
> If you have integer literals in your program, the compiler sees a
> fromInteger in front of them. So the value is just converted to type
> Int automatically, because that is expected here.
> You can give different numeric default declarations in your own
> modules. Please see sections 10.3 (for overloaded literals) and 10.4
> (for defaults) here:
> Sometimes you can get an overflow like this:
> Prelude> 100000000000000000000000 :: Int
> > 3) In main again, what does expression [| x |] mean ? Why this cannot be execute in
> > GHCi ?
> It's Template Haskell, and is used there for some kind of optimisation
> (I think). Template Haskell needs to be enabled with a command line
> switch for it to work. Please see the documentation for more
> information. It's section 7.6 in your User's Guide.
> Though in this case you can probably just remove it to try out the
> program. Perhaps someone else can explain what actual effect it has
I think it just computes a single function call to pureSieve at
compile time. I believe its origin is from making a point that when
you stop comparing apples to apples it's easy to cheat (this code
comes from a discussion on this list where someone insisted on adding
optimizations to the, admittedly naive, algorithm in C# and comparing
it without making the same optimizations in Haskell -- so someone, I
forget who but I'm a search will turn it up, wrote a quick template
Haskell "optimization" to make a point that we don't really get useful
results unless we compare the same algorithms in both languages).
In general you should probably ignore that TH bit and just call
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