7 ms·

I have a longer comment here that argues the opposite: list-comprehensions are only a syntactic pun or two away from set-builder notation, which is a higher lev

by frig 17y ago

I have a longer comment here that argues the opposite: list-comprehensions are only a syntactic pun or two away from set-builder notation, which is a higher level-of-abstraction (it declaratively states what it is) than map+filter (which specify a procedure to generate it, albeit at higher level of abstraction than a for-loop). If you can put together a nontrivial usage of map+filter with at least three source collections that's more concise than the equivalent list comprehension I'll (figuratively) eat my hat.

riobard 17y ago
I'm not so sure if set notation is higher level, but for the example in your longer comment, map/filter/product is not that bad if you use it wisely. Here is my version: map(lambda (w,s,l): {'widget': w, 'sprocket': s, 'location': l} filter(lambda (w,s,l): l.hasInStock(w) and l.hasInStock(s) and w.isUsableWith(s), product(widgets, sprockets, locations))) Well, I agree it is not any conciser that its list comprehension (about the same I guess?). Nothing is perfect, like you said, know you tool :)
- frig 17y ago
  You'll be thrilled (lol) to know that the lambda-tuple syntax isn't in python 3 (!); it does make my examples more concise. The argument in favor of set-notation being higher level is it's less specific (it doesn't explicitly provide a sequence of operations, just an outcome). List comprehensions look like set notation but have an implicit procedural translation you have to keep in mind to use them well, so it's a toss-up. I prefer map/filter/reduce when sequencing has large performance implications but for simple filtering or raw-data-shaping comprehensions read more smoothly. http://books.google.com/books?id=RvY5BM0Xt1wC&lpg=PT367&ots=m_Yzk8XnDQ&dq=python%20lambda%20tuple%20argument&pg=PT367#v=onepage&q=python%20lambda%20tuple%20argument&f=false http://books.google.com/books?id=RvY5BM0Xt1wC&lpg=PT367&...
  - riobard 17y ago
    > You'll be thrilled (lol) to know that the lambda-tuple syntax isn't in python 3 Now you know why many people like me are gradually pissed off by Python and start the exile ... currently trying Scala and it seems a nice language (with list comprehension too! :) > List comprehensions look like set notation but have an implicit procedural translation you have to keep in mind to use them well, so it's a toss-up. Actually I think that's the problem I have with list comprehensions: I use them a lot in my code, usually 1~3 levels nested and then have a hard time tracking down the order of implicit loops (which is inner vs outer) and make sure the intermediate variables (x for x in y for y in z ...) do not clash ... OK maybe I'm using it too much and in the wrong way :( > I prefer map/filter/reduce when sequencing has large performance implications but for simple filtering or raw-data-shaping comprehensions read more smoothly. I didn't know map/filter is faster than list comprehensions? I thought both are optimized by Python interpreter. But I like the idea of knowing that at least map can be parallelized easily. But since Python does not utilize multicore in a decent way, all bets are off :(
maxtilford 17y ago
Using do-notation is probably cheating, but what do you think of this? (in Haskell): do { w <- widgets; s <- sprockets; l <- locations; guard (l `hasInStock` w); guard (l `hasInStock` s); guard (w `isUsableWith` s); return (w, s, l); } We're trading horizontal space for vertical space. I think it's much clearer than either list comprehensions or plain map/filters. It's the best of both worlds.
- frig 17y ago
  I like the look of it. Am I correct that the order the statements are in translates into the order things are evaluated in when the code is called? If eg I edited it to be: do { l <- locations; w <- widgets; guard (l `hasInStock` w); s <- sprockets; guard (l `hasInStock` s); guard (w `isUsableWith` s); return (w,s,l); } Does that force it to go through "location-first" and only check the w and s of (w,s,l) for compatibility if it has already ascertained that w and s are in stock @ l?
  - maxtilford 17y ago
    Yes, that's correct.

by frig 17y ago

riobard 17y ago

I'm not so sure if set notation is higher level, but for the example in your longer comment, map/filter/product is not that bad if you use it wisely. Here is my version: map(lambda (w,s,l): {'widget': w, 'sprocket': s, 'location': l} filter(lambda (w,s,l): l.hasInStock(w) and l.hasInStock(s) and w.isUsableWith(s), product(widgets, sprockets, locations))) Well, I agree it is not any conciser that its list comprehension (about the same I guess?). Nothing is perfect, like you said, know you tool :)

frig 17y ago
You'll be thrilled (lol) to know that the lambda-tuple syntax isn't in python 3 (!); it does make my examples more concise. The argument in favor of set-notation being higher level is it's less specific (it doesn't explicitly provide a sequence of operations, just an outcome). List comprehensions look like set notation but have an implicit procedural translation you have to keep in mind to use them well, so it's a toss-up. I prefer map/filter/reduce when sequencing has large performance implications but for simple filtering or raw-data-shaping comprehensions read more smoothly. http://books.google.com/books?id=RvY5BM0Xt1wC&lpg=PT367&ots=m_Yzk8XnDQ&dq=python%20lambda%20tuple%20argument&pg=PT367#v=onepage&q=python%20lambda%20tuple%20argument&f=false http://books.google.com/books?id=RvY5BM0Xt1wC&lpg=PT367&...
- riobard 17y ago
  > You'll be thrilled (lol) to know that the lambda-tuple syntax isn't in python 3 Now you know why many people like me are gradually pissed off by Python and start the exile ... currently trying Scala and it seems a nice language (with list comprehension too! :) > List comprehensions look like set notation but have an implicit procedural translation you have to keep in mind to use them well, so it's a toss-up. Actually I think that's the problem I have with list comprehensions: I use them a lot in my code, usually 1~3 levels nested and then have a hard time tracking down the order of implicit loops (which is inner vs outer) and make sure the intermediate variables (x for x in y for y in z ...) do not clash ... OK maybe I'm using it too much and in the wrong way :( > I prefer map/filter/reduce when sequencing has large performance implications but for simple filtering or raw-data-shaping comprehensions read more smoothly. I didn't know map/filter is faster than list comprehensions? I thought both are optimized by Python interpreter. But I like the idea of knowing that at least map can be parallelized easily. But since Python does not utilize multicore in a decent way, all bets are off :(

maxtilford 17y ago

Using do-notation is probably cheating, but what do you think of this? (in Haskell): do { w <- widgets; s <- sprockets; l <- locations; guard (l `hasInStock` w); guard (l `hasInStock` s); guard (w `isUsableWith` s); return (w, s, l); } We're trading horizontal space for vertical space. I think it's much clearer than either list comprehensions or plain map/filters. It's the best of both worlds.

frig 17y ago
I like the look of it. Am I correct that the order the statements are in translates into the order things are evaluated in when the code is called? If eg I edited it to be: do { l <- locations; w <- widgets; guard (l `hasInStock` w); s <- sprockets; guard (l `hasInStock` s); guard (w `isUsableWith` s); return (w,s,l); } Does that force it to go through "location-first" and only check the w and s of (w,s,l) for compatibility if it has already ascertained that w and s are in stock @ l?
- maxtilford 17y ago
  Yes, that's correct.