On Thu, 2007-10-25 at 19:59 -0200, Maurício wrote:
Hi,
Today, if I write:
[a:[b] | a<-"ab" , b<-"12"]
I get:
["a1","a2","b1","b2"]
Are there any guarantees that I'll never get ["a1","b1","a2","b2"] instead, i.e., that the first list will always be the last one to be fully transversed? Even if I use a different compiler or a future version of Haskell?
Reading how list comprehensions are translated in the Haskell report it seems the answer is yes.
Correct.
Is that written in stone?
Yes. It's a consequence of the MonadPlus law (for [] and other non-determinism monads) (xn `mplus` ys) >>= f = (xn >>= f) `mplus` (ys >>= f) which implies [ f x y | x <- xn ++ xn', y <- ys] = [ f x y | x <- xn, y <- ys] ++ [ f x y | x <- xn', y <- ys] (This rule plus the monad laws plus the natural transformation law for return map f (return x) = return (f x) provides a complete calculation system for list comprehensions, btw. And those laws are all very much set in stone.)
Can compilers do it in their own different way?
No. jcc