Jan Brosius wrote:
> Doesn't haskell 98 allow in place updating e.g; for records?
Semantically yes, pragmatically no.
What this means is, it gives you what *looks* like in-place update (you
don't have to write the copying yourself), but in practice it is
usually implemented as copying... a new record is created with all
fields the same except for the one you changed.
How can you tell?
Look at the following program:
data Foo = Foo { a :: Int, b :: String }
instance Show Foo where
show f = show (a f) ++ " " ++ show (b f)
foo = Foo { a = 99, b = "green bottles\n" }
main = do print foo
let bar = foo { a = 98 }
print bar
Obviously, this displays
99 green bottles
98 green bottles
on stdout. It *looks* like you've updated the `a' field of foo. But
in fact, foo is still there, untouched: you could have written
main = do print foo
let bar = foo { a = 98 }
print bar
print foo
and got
99 green bottles
98 green bottles
99 green bottles
Of course, in the first program above, a clever compiler could figure
out you never use foo again and implement the update by true
update-in-place, but this is quite hard for the compiler to do.
Usually you give it a hand by saying `I will use this datum in a
single-threaded manner' by putting it inside the ST or IO monads. This
makes your code look much more imperative, though.
> Jan
HTH.
--KW 8-)
--
: Keith Wansbrough, MSc, BSc(Hons) (Auckland) ------------------------:
: PhD Student, Computer Laboratory, University of Cambridge, England. :
: (and recently of the University of Glasgow, Scotland. [><] ) :
: Native of Antipodean Auckland, New Zealand: 174d47' E, 36d55' S. :
: http://www.cl.cam.ac.uk/users/kw217/ mailto:[EMAIL PROTECTED] :
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--
: Keith Wansbrough, MSc, BSc(Hons) (Auckland) ------------------------:
: PhD Student, Computer Laboratory, University of Cambridge, England. :
: (and recently of the University of Glasgow, Scotland. [><] ) :
: Native of Antipodean Auckland, New Zealand: 174d47' E, 36d55' S. :
: http://www.cl.cam.ac.uk/users/kw217/ mailto:[EMAIL PROTECTED] :
:---------------------------------------------------------------------:
