Erik:
You have discovered the essence of monads, ie the difference between the bad
and ugly world of side-effecting computations and the nice and clean world
of pure functions. And even using my favourite example (*)!
Let's put it in other words: I knew the difference,
On Fri, 28 Apr 2000, Fergus Henderson wrote:
This is all fine and dandy if `currentSecond' is within `where'
clause, because it will be always evaluated afresh.
It might happen to work with current Haskell implementations,
but I don't think there's any guarantee of that.
Since we are in a mood for puzzles today ..
How to update file "xxx" without
making backup "yyy" first, as in:
readFile "xxx" =
writeFile "yyy" =
readFile "yyy" =
process =
On Thu, 27 Apr 2000 Jan Skibinski wrote:
Since we are in a mood for puzzles today ..
How to update file "xxx" without
making backup "yyy" first, as in:
readFile "xxx" =
writeFile "yyy" =
readFile "yyy" =
process
Ralf Krueger wrote:
I think, the problem can be solved by using the IO library and some
strictness:
import IO
main = do text - readFile "xxx"
text' - process $! text
file - openFile "xxx" WriteMode
hPutStr file text'
hClose file
seems
Ralf and Sven:
Thank you both for your answers. I knew that strictness
was needed here, but I was seeking some elegant solution.
I prefer your answer, Sven, a bit more. Could you elaborate
on your `hack' a bit more? It seems to be a good topic for
Thu, 27 Apr 2000 19:20:57 +0200, Ralf Krueger [EMAIL PROTECTED]
pisze:
main = do text - readFile "xxx"
text' - process $! text
$! will usually not suffice, because it forces only the beginning of
the list.
"foldr seq (return ()) text" should suffice.
Or use a GHC extension:
On 27 Apr 2000, Marcin 'Qrczak' Kowalczyk wrote:
Unless we are talking about unsafe extensions, which OTOH are very
useful too. Sometimes eliminating unsafePerformIO would require
huge rewrite of the whole program, making it less readable and less
efficient. But they should be clearly
On 27-Apr-2000, Jan Skibinski [EMAIL PROTECTED] wrote:
currentSecond = second $ unsafePerformIO localDateTime
where `localDateTime' has been defined via primitive
call to C:
localDateTime :: IO DateTime
To my distress the clock stopped after the
