4 ms·

You get undef, that's part of the reason we added an undef concept (it's a value that isn't a value, though if you treat it like an int I believe it is zero, li

You get undef, that's part of the reason we added an undef concept (it's a value that isn't a value, though if you treat it like an int I believe it is zero, like a string and you get "").

IshKebab 6y ago
So there's `undef` and `null`? I wonder if there's a popular language that made the same mistake you could have learnt from :-P
- luckydude 6y ago
  There is no null, only undef. It's not a value, there is no value that means undef. We implemented by stealing the high order bit from the reference counter that tcl uses for garbage collection. If it is set, the variable undefined, if it isn't, it's a normal variable. I thought that was clever given that tcl's reference counter was signed and it only uses positive values. So we made it unsigned and got the bit for free.
  - skybrian 6y ago
    What happens if you call a Tcl function with an undef argument? Is this running Tcl code on a slightly different runtime, or do they interop in some other way?
    - luckydude 6y ago
      In Tcl, everything is a string. The string rep of undef is "" so Tcl gets "" but doesn't know that it is undef. So yeah, there is some sloppiness there. Tcl tends to use "" as sort of a null or undef so lots of stuff just works but no promises on that. This is sloppy simply because the Tcl die hards (are there any left?) refused to see the value of undef, a value for a variable that said there is no value. We used it all over the place, there clearly is value. They didn't see that, Little never got pushed back into the Tcl source base so Tcl never thought about undef. It is what it is, we live in an imperfect world. I tried.
CyberDildonics 6y ago
I think that is a giant design mistake since you are pushing the discovery of an error to some place else in the program, potentially very far from where the error occurred.
- luckydude 6y ago
  Without providing an example of how to do it better, I'm not sure I see your point. Yeah, I see the problem you point out. Consider char *p = 0; some complicated code that should have set p but didn't... char c = *p; // SEGV So do you have a way to push that error all the way back to wherever you think it should be pushed?
  - CyberDildonics 6y ago
    The person you replied to asked what the behavior was when you access an offset of an array that doesn't exist and you are giving me an example of dereferencing a null pointer. The array access should error out on the line that it happens. The dereference could print a line of the last assignment, but this isn't what was being talked about.
    - luckydude 6y ago
      I don't see the array access as any different than a null pointer dereference, in both cases you are asking for something that isn't there. In Tcl, stuff just gets auto expanded on assignment and returns "" (I think) on dereference. We chose to return undef. I'm not sure what the problem is.
      - CyberDildonics 6y ago
        I explained the problem already (the error does not show up where it actually happens). I also explained a huge difference between the two already (you know how large an array is and can detect an out of bounds access when it happens).
        6y ago

You get undef, that's part of the reason we added an undef concept (it's a value that isn't a value, though if you treat it like an int I believe it is zero, like a string and you get "").

IshKebab 6y ago

So there's `undef` and `null`? I wonder if there's a popular language that made the same mistake you could have learnt from :-P

luckydude 6y ago
There is no null, only undef. It's not a value, there is no value that means undef. We implemented by stealing the high order bit from the reference counter that tcl uses for garbage collection. If it is set, the variable undefined, if it isn't, it's a normal variable. I thought that was clever given that tcl's reference counter was signed and it only uses positive values. So we made it unsigned and got the bit for free.
- skybrian 6y ago
  What happens if you call a Tcl function with an undef argument? Is this running Tcl code on a slightly different runtime, or do they interop in some other way?
  - luckydude 6y ago
    In Tcl, everything is a string. The string rep of undef is "" so Tcl gets "" but doesn't know that it is undef. So yeah, there is some sloppiness there. Tcl tends to use "" as sort of a null or undef so lots of stuff just works but no promises on that. This is sloppy simply because the Tcl die hards (are there any left?) refused to see the value of undef, a value for a variable that said there is no value. We used it all over the place, there clearly is value. They didn't see that, Little never got pushed back into the Tcl source base so Tcl never thought about undef. It is what it is, we live in an imperfect world. I tried.

CyberDildonics 6y ago

I think that is a giant design mistake since you are pushing the discovery of an error to some place else in the program, potentially very far from where the error occurred.

luckydude 6y ago
Without providing an example of how to do it better, I'm not sure I see your point. Yeah, I see the problem you point out. Consider char *p = 0; some complicated code that should have set p but didn't... char c = *p; // SEGV So do you have a way to push that error all the way back to wherever you think it should be pushed?
- CyberDildonics 6y ago
  The person you replied to asked what the behavior was when you access an offset of an array that doesn't exist and you are giving me an example of dereferencing a null pointer. The array access should error out on the line that it happens. The dereference could print a line of the last assignment, but this isn't what was being talked about.
  - luckydude 6y ago
    I don't see the array access as any different than a null pointer dereference, in both cases you are asking for something that isn't there. In Tcl, stuff just gets auto expanded on assignment and returns "" (I think) on dereference. We chose to return undef. I'm not sure what the problem is.
    - CyberDildonics 6y ago
      I explained the problem already (the error does not show up where it actually happens). I also explained a huge difference between the two already (you know how large an array is and can detect an out of bounds access when it happens).
      - 6y ago