5 ms·

Ruby’s Hashes and Perl’s Autovivification, in JavaScript

36 points by braythwayt 8y ago

gvx 8y ago
> Our third use case involves checking whether the defaultValue is an ordinary value or a function. We could check every time it’s accessed, but instead we’ll assign different function bodies to the proxy’s get key. That way, it’s only checked at (open air quotes) compile time (close air quotes): The code below that paragraph actually does check on every access, though.
- braythwayt 8y ago
  Hah, yes, I made that change mid-way through to allow for Ruby’s feature where you can mutate the default value of a Hash (a feature I personally have never used), but didn’t update the text. Fix on the way. Thanks for spotting the discrepancy!
- fenomas 8y ago
  > The code below that paragraph actually does check on every access, though. I could be wrong, but I think a sufficiently clever JS engine (like v8) would remove that check at runtime, by realizing that half of the ternary operator is dead code. (If it's hot code and it gets optimized, anyway).
  - braythwayt 8y ago
    Ah, the mythical “sufficiently smart compiler” :-)
    - fenomas 8y ago
      To clarify I wasn't being hypothetical, I think v8 will optimize out that check today. I phrased it vaguely because I'm not sure whether other engines would as well.
      - braythwayt 8y ago
        It’s certainly doable. The `defaultValue` parameter is passed by value, so it will never be rebound. The result of using `typeof` on a value never changes in JS, even if the value is modified. So... it should be possible to optimize the check away after the first time.
tkahnoski 8y ago
I have a list of Perl ‘wtf’ moments. The discovery of autovification was one of them. Perl autovivifies on existence checks. This has led to A LOT of bugs. Particularly since native Perl objects have all the behaviors of hashes so instead of getting the equivalent of a null exception Perl happily creates a hash and the process won’t bomb out until I try to call a method on the hash which I was expecting as a particular type. In legacy code, this has led to a lot of safety checks verifying the variable is actually the expected type.
- braythwayt 8y ago
  Autovivifying variables is wildly out there. I feel your pain! Autovivifying properties of a dictionary could certaintly lead to some bugs, but as presented here, checking for existence won’t invoke autovivification.
  - tkahnoski 8y ago
    Indeed. I wish with Perl I had a way to be explicit about when I could disable this behavior but its builtin to the language to such a degree to be unavoidable. (Using different Perl object frameworks and getter/setter methods is a possibility to protect against this class of error) The implementation provided in the article is at least a known property of that particular object type and not a property of all objects.
    - draegtun 8y ago
      You can switch off or amend autovivification behaviour using this pragma module - https://metacpan.org/pod/autovivification https://metacpan.org/pod/autovivification no autovivification; # switches off autoviv except for setting no autovivification 'exists'; # just for exists checks
      - tkahnoski 8y ago
        OMG. All I have to say is thank you.
Ultimatt 8y ago
If anyone's wondering how you get autoviv in Python, its surprisingly gross/elegant/fun depending on how much you use the phrasing "pythonic" in your interaction with people. some_autoviv_variable = defaultdict(lambda: defaultdict(dict))
- braythwayt 8y ago
  Cool! Does that create defaultdicts “all the way down?”
  - Ultimatt 8y ago
    At least until it hits the turtle bedrock at the end of your system memory. Oh maybe you need the inner one to be a lambda of defaultdict actually, then it will. turtles = lambda: defaultdict(turtles) some_autoviv_variable = defaultdict(turtles) Have to confess I'm a polyglot type person, and even Im starting to twitch at "unpythonic" feels of having something as nuts as a recursive expression in a lambda with some assignment.
    - klibertp 8y ago
      I could agree with "unpythonic", as Python is a bit weak in this department, but "recursive lambda" being "nuts" is a bit much - especially here, given the site's name ;)
      - Ultimatt 8y ago
        Oh yeah only nuts given we are in Python land. It's kind of like if some super advanced alien artefact landed on Earth. It would appear as magic incongruent with reality. Thats Python and almost anything thats expressive. God forbid your brain can imagine something beyond simple statements.
    - braythwayt 8y ago
      > Im starting to twitch at "unpythonic" feels of having something as nuts as a recursive expression in a lambda with some assignment. Welcome to the Dark Side. Enjoy the cookies!
mickeyben 8y ago
Great article! I had some fun with Autovivification a couple of years ago and implemented a small ruby library to do just that https://github.com/mickey/black-hole-struct https://github.com/mickey/black-hole-struct. I think the whole point was to help generate a few very long json files (http://docs.grafana.org/reference/dashboard/#dashboard-json http://docs.grafana.org/reference/dashboard/#dashboard-json) but in the end I had way more fun with this new concept!

36 points by braythwayt 8y ago

gvx 8y ago

> Our third use case involves checking whether the defaultValue is an ordinary value or a function. We could check every time it’s accessed, but instead we’ll assign different function bodies to the proxy’s get key. That way, it’s only checked at (open air quotes) compile time (close air quotes): The code below that paragraph actually does check on every access, though.

braythwayt 8y ago
Hah, yes, I made that change mid-way through to allow for Ruby’s feature where you can mutate the default value of a Hash (a feature I personally have never used), but didn’t update the text. Fix on the way. Thanks for spotting the discrepancy!
fenomas 8y ago
> The code below that paragraph actually does check on every access, though. I could be wrong, but I think a sufficiently clever JS engine (like v8) would remove that check at runtime, by realizing that half of the ternary operator is dead code. (If it's hot code and it gets optimized, anyway).
- braythwayt 8y ago
  Ah, the mythical “sufficiently smart compiler” :-)
  - fenomas 8y ago
    To clarify I wasn't being hypothetical, I think v8 will optimize out that check today. I phrased it vaguely because I'm not sure whether other engines would as well.
    - braythwayt 8y ago
      It’s certainly doable. The `defaultValue` parameter is passed by value, so it will never be rebound. The result of using `typeof` on a value never changes in JS, even if the value is modified. So... it should be possible to optimize the check away after the first time.

tkahnoski 8y ago

I have a list of Perl ‘wtf’ moments. The discovery of autovification was one of them. Perl autovivifies on existence checks. This has led to A LOT of bugs. Particularly since native Perl objects have all the behaviors of hashes so instead of getting the equivalent of a null exception Perl happily creates a hash and the process won’t bomb out until I try to call a method on the hash which I was expecting as a particular type. In legacy code, this has led to a lot of safety checks verifying the variable is actually the expected type.

braythwayt 8y ago
Autovivifying variables is wildly out there. I feel your pain! Autovivifying properties of a dictionary could certaintly lead to some bugs, but as presented here, checking for existence won’t invoke autovivification.
- tkahnoski 8y ago
  Indeed. I wish with Perl I had a way to be explicit about when I could disable this behavior but its builtin to the language to such a degree to be unavoidable. (Using different Perl object frameworks and getter/setter methods is a possibility to protect against this class of error) The implementation provided in the article is at least a known property of that particular object type and not a property of all objects.
  - draegtun 8y ago
    You can switch off or amend autovivification behaviour using this pragma module - https://metacpan.org/pod/autovivification https://metacpan.org/pod/autovivification no autovivification; # switches off autoviv except for setting no autovivification 'exists'; # just for exists checks
    - tkahnoski 8y ago
      OMG. All I have to say is thank you.

Ultimatt 8y ago

If anyone's wondering how you get autoviv in Python, its surprisingly gross/elegant/fun depending on how much you use the phrasing "pythonic" in your interaction with people. some_autoviv_variable = defaultdict(lambda: defaultdict(dict))

braythwayt 8y ago
Cool! Does that create defaultdicts “all the way down?”
- Ultimatt 8y ago
  At least until it hits the turtle bedrock at the end of your system memory. Oh maybe you need the inner one to be a lambda of defaultdict actually, then it will. turtles = lambda: defaultdict(turtles) some_autoviv_variable = defaultdict(turtles) Have to confess I'm a polyglot type person, and even Im starting to twitch at "unpythonic" feels of having something as nuts as a recursive expression in a lambda with some assignment.
  - klibertp 8y ago
    I could agree with "unpythonic", as Python is a bit weak in this department, but "recursive lambda" being "nuts" is a bit much - especially here, given the site's name ;)
    - Ultimatt 8y ago
      Oh yeah only nuts given we are in Python land. It's kind of like if some super advanced alien artefact landed on Earth. It would appear as magic incongruent with reality. Thats Python and almost anything thats expressive. God forbid your brain can imagine something beyond simple statements.
  - braythwayt 8y ago
    > Im starting to twitch at "unpythonic" feels of having something as nuts as a recursive expression in a lambda with some assignment. Welcome to the Dark Side. Enjoy the cookies!

mickeyben 8y ago

Great article! I had some fun with Autovivification a couple of years ago and implemented a small ruby library to do just that https://github.com/mickey/black-hole-struct https://github.com/mickey/black-hole-struct. I think the whole point was to help generate a few very long json files (http://docs.grafana.org/reference/dashboard/#dashboard-json http://docs.grafana.org/reference/dashboard/#dashboard-json) but in the end I had way more fun with this new concept!