On Wed, Jan 30, 2013 at 7:02 AM, Junior White
Thanks for your reply! I must learn more to fully understand what's going on inside the list comprehension. But when I frist learn Haskell, it says sequence doesn't matter, but now it is a big matter, can compiler do some thing for us? I think this behavior is not friendly to newbies like me, I will take a very long time to work through it.
No, the compiler can't help you here. The compiler is not an oracle; even if it could invert your calculation (effectively swapping the loops around), it can't know which one is more appropriate. As to sequences: sequence doesn't matter indeed; data dependencies matter, and loop ordering imposes a data dependency because loops in Haskell are encoded as data structures (lists). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net