Re: [Haskell-cafe] Re: evaluation semantics of bind

On Fri, 2009-02-06 at 00:51 +0100, Peter Verswyvelen wrote:

On Thu, Feb 5, 2009 at 8:20 PM, ChrisK wrote: Since this is strict there is no laziness and the code must evaluate the input and output "State RealWorld" to ensure they are not bottom or error.

Interesting. I also thought it was the passing of the RealWorld that caused the sequencing, I never realized that the strictness played an important role here.

So what would happen if it would be lazy instead of strict? What kind of craziness would occur?

The order of side effects would be demand-driven, rather than order-of-statement driven. So if I said: do x <- getChar y <- getChar z <- getChar then in the sequel, the first character I evaluated would be the first character read. Of course, Haskell's order of evaluation is undefined, so whether the string read from STDIN was [x, y, z] or [y, x, z] or [z, x, y] or some other permutation would be undefined. And, of course, if I never evaluated y at all, y would never be read --- there would be only two characters of input. Essentially, the program would act as if every statement was wrapped up in an unsafeInterleaveIO. jcc