3 ms·

I guess I should have been more clear. I'm aware that they do not add extra passes in some languages. But in Swift, C++, Common Lisp, OCaml, Python, Ruby, Per

I guess I should have been more clear. I'm aware that they do not add extra passes in some languages. But in Swift, C++, Common Lisp, OCaml, Python, Ruby, Perl, Java, Scheme, and many others, adding another map/filter/etc. will add another pass through the sequence unless you're going out of the way to avoid it.

dragonwriter 11y ago
> But in Swift, C++, Common Lisp, OCaml, Python, Ruby, Perl, Java, Scheme, and many others, adding another map/filter/etc. will add another pass through the sequence unless you're going out of the way to avoid it. I think this is wrong in the case of Python -- map/filter/reduce are lazy, and so chaining them does not result in multiple iterations of the source sequence. In Ruby, its definitely sometimes wrong, because whether or not iteration is lazy depends on the particular Enumerable object; and, in the worst case, in Ruby, you don't have to go very far out of your way. In the event that the Enumerable you are operating on isn't already a lazy one, you just call its lazy method, and chain your operations after that.
oisdk 11y ago
In Swift: (0...100) .filter { $0 % 2 == 0 } .map { $0 * 2 } Will pass through the sequence multiple times, yes. But to avoid it it's as easy as: (0...100) .lazy .filter { $0 % 2 == 0 } .map { $0 * 2 }
ta0967 11y ago
how much of a performance drop have you measured in real world? compare my @judges = (); for my $sport (@sports) { if (is_watersport $sport) { push @judges, get_judges $sport; } } to my @judges = map get_judges, grep is_watersport, @sports; is it worth fretting over an added pass? how many sports are we dealing with anyway? and IME, nested loops induce errors in the form of accidental `O(N*M)`.
- jlarocco 11y ago
  Your example is a straw man. IRL, I've seen this crop up while processing database results and large datasets. On several occasions I've shaved up to several minutes off of execution times by replacing chained map, filter, and zip calls that were iterating through the data multiple times with more complicated expressions that only performed the iteration once. I've also written a graphics applications where converting several chained map/filters into a single pass made interacting with the application noticeably less laggy. I'm actually surprised so many people have challenged this as a potential pitfall. I didn't pull it out of the air or from trivial examples. I've seen it happen.
sanderjd 11y ago
Just to add to the chorus of "well, maybe not by default, but it's pretty easy to do", the streams library in Java 8 is lazy, and there's this library for C++14[0]. [0]: https://github.com/jscheiny/Streams https://github.com/jscheiny/Streams.

dragonwriter 11y ago

> But in Swift, C++, Common Lisp, OCaml, Python, Ruby, Perl, Java, Scheme, and many others, adding another map/filter/etc. will add another pass through the sequence unless you're going out of the way to avoid it. I think this is wrong in the case of Python -- map/filter/reduce are lazy, and so chaining them does not result in multiple iterations of the source sequence. In Ruby, its definitely sometimes wrong, because whether or not iteration is lazy depends on the particular Enumerable object; and, in the worst case, in Ruby, you don't have to go very far out of your way. In the event that the Enumerable you are operating on isn't already a lazy one, you just call its lazy method, and chain your operations after that.

oisdk 11y ago

In Swift: (0...100) .filter { $0 % 2 == 0 } .map { $0 * 2 } Will pass through the sequence multiple times, yes. But to avoid it it's as easy as: (0...100) .lazy .filter { $0 % 2 == 0 } .map { $0 * 2 }

ta0967 11y ago

how much of a performance drop have you measured in real world? compare my @judges = (); for my $sport (@sports) { if (is_watersport $sport) { push @judges, get_judges $sport; } } to my @judges = map get_judges, grep is_watersport, @sports; is it worth fretting over an added pass? how many sports are we dealing with anyway? and IME, nested loops induce errors in the form of accidental `O(N*M)`.

jlarocco 11y ago
Your example is a straw man. IRL, I've seen this crop up while processing database results and large datasets. On several occasions I've shaved up to several minutes off of execution times by replacing chained map, filter, and zip calls that were iterating through the data multiple times with more complicated expressions that only performed the iteration once. I've also written a graphics applications where converting several chained map/filters into a single pass made interacting with the application noticeably less laggy. I'm actually surprised so many people have challenged this as a potential pitfall. I didn't pull it out of the air or from trivial examples. I've seen it happen.

sanderjd 11y ago

Just to add to the chorus of "well, maybe not by default, but it's pretty easy to do", the streams library in Java 8 is lazy, and there's this library for C++14[0]. [0]: https://github.com/jscheiny/Streams https://github.com/jscheiny/Streams.