[Haskell-cafe] Haskell and "memoization"

[Ozgur Akgun]

Sorry for the last mail, I now tried it and it returns the next value every
time I call it.

I was using an unsafeperformIO trick somewhere, and that fas the one
resulting in the previously described behaviour.

You can just ignore the previous mail.

2009/12/17 Ozgur Akgun <ozgurakgun at gmail.com>

> Maybe not related, but does the following prove next is called once and
> only once.
>
>
> import qualified Data.ByteString as BS
> import qualified Data.ByteString.Char8 as BSC
>
> next =
>     do
>         nextcache <- BS.readFile "next.cache"
>         let nextint = readInt (BSC.unpack nextcache)
>         BS.writeFile "next.cache" (BSC.pack (show (nextint+1)))
>         return nextint
>
> readInt :: String -> Int
> readInt = read
>
>
> I put a single character, 1 in the file "next.cache" when I run this
> through ghci, and call next several times, I always get a 1. Whereas in the
> file there is a 2.
> I see that next is a trial of creating a function which returns different
> things everytime its called, but it's in the IO monad, so that should be
> doable.
>
> When I re-run ghci, now it starts to give 2 everytime I call it. Does that
> mean, it doesn't bother to re-read the file while we are in the same
> process.
>
>
> Hope it relates to the OP's question in some way :)
>
> Best,
>
> 2009/12/16 Daniel Fischer <daniel.is.fischer at web.de>
>
> Am Mittwoch 16 Dezember 2009 15:49:54 schrieb michael rice:
>> > Thanks all,
>> >
>> > OK, so this definition of fib
>> >
>> > fib 0 = 1
>> > fib 1 = 1
>> > fib n = fib (n-1) + fib (n-2)
>> >
>> > would involve a lot of recomputation for some large n,
>>
>> Where "large" can start as low as 20; 60 would be out of reach.
>>
>> > which memoization would eliminate?
>>
>> Right.
>> >
>> > Michael
>>
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>>
>
>
>
> --
> Ozgur Akgun
>



-- 
Ozgur Akgun
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