[Haskell-cafe] One-shot? (was: Global variables and stuff)

Keith Wansbrough Keith.Wansbrough at cl.cam.ac.uk
Thu Nov 11 11:07:05 EST 2004

Graham Klyne wrote:

> At 12:27 11/11/04 +0000, Ben Rudiak-Gould wrote:
> >going to be safe, because it's just not the case that
> >
> >    x = once (newIORef ())
> >    y = x
> >
> >has the same intended meaning as
> >
> >    x = once (newIORef ())
> >    y = once (newIORef ())
> >
> >No amount of compiler-specific magic is going to fix this.
> Ah, yes, I take the point now.
> Isn't this generally the case for any value in the IO monad?  (Brushing a 
> murky area of equivalence;  the same IO computation used twice may yield 
> different results, so I'm not clear to what extent it is meaningful to say 
> that any IO value is the same as any other, including itself, in any 
> observable sense.)

No.  "getChar" is always "the IO operation that reads a character from
stdin".  You can always substitute one instance of "getChar" for
another; you can even say "foo = getChar" and substitute "foo" for
every occurrence of "getChar".  A value of type IO a is a
*computation*; its result may change, but the computation itself

--KW 8-)

More information about the Haskell-Cafe mailing list