[Haskell-cafe] A few questions on primes generating.

[Pekka Karjalainen]

On 8/13/07, L.Guo <leaveye.guo at gmail.com> wrote:
> Hi All:

Hello,

>
> 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)
46340.950001051984

> 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:
http://www.haskell.org/tutorial/numbers.html

Sometimes you can get an overflow like this:

Prelude> 100000000000000000000000 :: Int
-159383552

> 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
here.

Pekka