undefined | Better HN

0 pointsIshKebab5y ago0 comments

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

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luckydude5y 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.

skybrian5y 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?

luckydude5y 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.

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luckydude5y ago

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.

skybrian5y 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?

luckydude5y 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.

It is what it is, we live in an imperfect world. I tried.

j / k navigate · click thread line to collapse