On Wed, Jan 30, 2013 at 7:02 AM, Junior White <efiish@gmail.com> wrote:

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

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