4 ms·

Sharp Variables

32 points by philogb 18y ago

Eliezer 18y ago
Sharp variables sound pretty dangerous. Has anyone hurt themselves using them?
lsb 18y ago
Incidentally, it's the same in Ruby, and supports : a = [1,2,3] => [1, 2, 3] a << a => [1, 2, 3, [...]] a.inspect It's not true laziness, though; it's as long as the list when you tie it to itself.
TweedHeads 18y ago
Everytime I see somebody using .toSource() I show due respect.
jcl 18y ago
The article asserts that sharp variables are "pretty useful" without actually mentioning a use. What exactly would one use them for?
- Deestan 18y ago
  It results in a human readable "pickle" format, which is its own reward, I believe.
- ken 18y ago
  I've seen them used to create inline infinite lists: (loop for loop-this in '#1=(:a :b :c . #1#) for just-once in '(1 2 3 4 5 6 7) do (format t "~S => ~S~%" just-once loop-this)) => 1 => :A 2 => :B 3 => :C 4 => :A 5 => :B 6 => :C 7 => :A NIL This encapsulates the "loop over this list forever" at its definition, instead of making me use an array and using length/modulo where it's used. There are of course many other solutions, but this one is both short and perfectly describes what I want. Many Lisp library functions take lists as inputs (no kidding!), so this is a very efficient alternative to actually creating a repetitive list. (Pythoners: think itertools.cycle). Also, any time you want to build a simple-but-not-trivial graph inline (great for unit testing), these come in handy.
- herdrick 18y ago
  Graphs. http://news.ycombinator.com/item?id=549194 http://news.ycombinator.com/item?id=549194
amalcon 18y ago
So a "sharp variable" is just a particular syntax to represent a reference? Is anyone else somewhat unimpressed by this? I mean, sure, it's kind of helpful for readability in the printout, and it's kind of nice to be able to one-line a recursive data structure. Just about any reasonably clear syntax will work for the former; there's no reason that the programmer would need to know about it before encountering it. The latter is mostly useful in a lazy evaluation context; in other situations, you're only rarely creating recursive data structures in the first place (circular linked lists are the most obvious case).
- jcl 18y ago
  I'm also struggling to see the big benefit. I guess if you are allowed multiple sharp variable references -- as apparently Mozilla's Javascript allows -- you can construct a directed graph in a single statement, including cycles. I don't see that being very useful outside of entering test cases, though.
  - herdrick 18y ago
    Graphs are the new hashtable - this is a great feature. Much of the interesting things being done now are done with giant graph structures. BTW, it's so lame that PLT Scheme is going in the opposite direction. They had the same excellent notation for a long time, but now since they forbid list mutation* they are useless. (* Yes, I know you can explicitly use mutable lists, but they don't work with the huge list library, for example SRFI-1, and there's no literal syntax for them, so it's like using lists in a language that doesn't like lists. Useless.) Actually, I wonder if there's a better syntax for graph literals than this? That would be valuable.
lacker 18y ago
Sharp variables don't seem particularly useful, but I didn't know about .toSource() in Firefox before I read this article, and that does sound like it could be useful.
onedognight 18y ago
Interesting, but I don't see mention of this in Javascript spec (http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf http://www.ecma-international.org/publications/files/ECMA-ST...), so I'm guessing it's not portable.
- jcl 18y ago
  It seems to be a Mozilla-only thing: https://developer.mozilla.org/en/Sharp_variables_in_JavaScript https://developer.mozilla.org/en/Sharp_variables_in_JavaScri...
shaunxcode 18y ago
I was hoping this was further evidence of javascripts roots as a lisp. But it seems like it is spider-monkey-centric and thus not some throwback to the original spec/implementation. Still cool!
10ren 18y ago
There must not be a space between the "#1" and the "=" - it behaves as one token, "#1=". Otherwise you get an "Illegal Character" error. Playing with it in Firefox's error console (is there an easier way?), it seems general. e.g. this works: var myArray = [1, 2, #1= [1,2,#1#]]; myArray.toSource(); The sharp variables seem to work as if you generated them as incrementing labels. Hence, the references must be back-references not forward-references (so you can say "[1, #1= [2, 3], #1#]", but not "[1, #1#, #1= [2, 3]]"). And they must be numeric ("[1, #a= [2, 3], #a#]" is out). And their naming becomes the order they are needed in (so "#2= [#1= [#2#], #1#]" becomes "#1=[#2=[#1#], #2#]"). Reminiscent of regex backreferences.

32 points by philogb 18y ago

Sharp variables sound pretty dangerous. Has anyone hurt themselves using them?

lsb 18y ago

Incidentally, it's the same in Ruby, and supports : a = [1,2,3] => [1, 2, 3] a << a => [1, 2, 3, [...]] a.inspect It's not true laziness, though; it's as long as the list when you tie it to itself.

TweedHeads 18y ago

Everytime I see somebody using .toSource() I show due respect.

jcl 18y ago

The article asserts that sharp variables are "pretty useful" without actually mentioning a use. What exactly would one use them for?

Deestan 18y ago
It results in a human readable "pickle" format, which is its own reward, I believe.
ken 18y ago
I've seen them used to create inline infinite lists: (loop for loop-this in '#1=(:a :b :c . #1#) for just-once in '(1 2 3 4 5 6 7) do (format t "~S => ~S~%" just-once loop-this)) => 1 => :A 2 => :B 3 => :C 4 => :A 5 => :B 6 => :C 7 => :A NIL This encapsulates the "loop over this list forever" at its definition, instead of making me use an array and using length/modulo where it's used. There are of course many other solutions, but this one is both short and perfectly describes what I want. Many Lisp library functions take lists as inputs (no kidding!), so this is a very efficient alternative to actually creating a repetitive list. (Pythoners: think itertools.cycle). Also, any time you want to build a simple-but-not-trivial graph inline (great for unit testing), these come in handy.
herdrick 18y ago
Graphs. http://news.ycombinator.com/item?id=549194 http://news.ycombinator.com/item?id=549194

amalcon 18y ago

So a "sharp variable" is just a particular syntax to represent a reference? Is anyone else somewhat unimpressed by this? I mean, sure, it's kind of helpful for readability in the printout, and it's kind of nice to be able to one-line a recursive data structure. Just about any reasonably clear syntax will work for the former; there's no reason that the programmer would need to know about it before encountering it. The latter is mostly useful in a lazy evaluation context; in other situations, you're only rarely creating recursive data structures in the first place (circular linked lists are the most obvious case).

jcl 18y ago
I'm also struggling to see the big benefit. I guess if you are allowed multiple sharp variable references -- as apparently Mozilla's Javascript allows -- you can construct a directed graph in a single statement, including cycles. I don't see that being very useful outside of entering test cases, though.
- herdrick 18y ago
  Graphs are the new hashtable - this is a great feature. Much of the interesting things being done now are done with giant graph structures. BTW, it's so lame that PLT Scheme is going in the opposite direction. They had the same excellent notation for a long time, but now since they forbid list mutation* they are useless. (* Yes, I know you can explicitly use mutable lists, but they don't work with the huge list library, for example SRFI-1, and there's no literal syntax for them, so it's like using lists in a language that doesn't like lists. Useless.) Actually, I wonder if there's a better syntax for graph literals than this? That would be valuable.

lacker 18y ago

Sharp variables don't seem particularly useful, but I didn't know about .toSource() in Firefox before I read this article, and that does sound like it could be useful.

onedognight 18y ago

Interesting, but I don't see mention of this in Javascript spec (http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf http://www.ecma-international.org/publications/files/ECMA-ST...), so I'm guessing it's not portable.

jcl 18y ago
It seems to be a Mozilla-only thing: https://developer.mozilla.org/en/Sharp_variables_in_JavaScript https://developer.mozilla.org/en/Sharp_variables_in_JavaScri...

shaunxcode 18y ago

I was hoping this was further evidence of javascripts roots as a lisp. But it seems like it is spider-monkey-centric and thus not some throwback to the original spec/implementation. Still cool!

10ren 18y ago

There must not be a space between the "#1" and the "=" - it behaves as one token, "#1=". Otherwise you get an "Illegal Character" error. Playing with it in Firefox's error console (is there an easier way?), it seems general. e.g. this works: var myArray = [1, 2, #1= [1,2,#1#]]; myArray.toSource(); The sharp variables seem to work as if you generated them as incrementing labels. Hence, the references must be back-references not forward-references (so you can say "[1, #1= [2, 3], #1#]", but not "[1, #1#, #1= [2, 3]]"). And they must be numeric ("[1, #a= [2, 3], #a#]" is out). And their naming becomes the order they are needed in (so "#2= [#1= [#2#], #1#]" becomes "#1=[#2=[#1#], #2#]"). Reminiscent of regex backreferences.