[Haskell-cafe] Re: [Haskell] Re: Haskell Digest, Vol 52, Issue 1

5 Dec 2007


      Taral wrote:
...
On 12/4/07, Simon Marlow  wrote:
...
do
     x <- newIVar
     let y = readIVar x
     writeIVar x 3
     print y
(I wrote the let to better illustrate the problem, of course you can inline
y if you want).  Now suppose the compiler decided to evaluate y before the
writeIVar.  What's to prevent it doing that?
Look at the translation:
newIVar >>= (\x -> let y = readIVar x in writeIVar x 3 >> print y)
y can't be floated out because it depends on x.
y doesn't need to be floated out, just evaluated eagerly.

Cheers,
	Simon