4 ms·

The Novelties of Lua 5.2 Roberto Ierusalimschy (pdf slideshow)

35 points by telemachos 15y ago

telemachos 15y ago
The attitude towards goto really struck a chord with me. It's "powerful" and "easy to explain". It's also "simple" to add to the VM. It's in, and purity be damned. There's something to be said here about pragmatism versus purity. Maybe also a comparison with the recent posts about OO and Rails. I'm hand-waving. I mean to write something longer about this, but I'm still trying to decide what exactly I make of it.
- justincormack 15y ago
  A lot of people wanted it is Lua is quite widely used as a code generation target, and generating gotos is useful often to match other language constructs.
watmough 15y ago
For anyone wondering about "C upvalues", which wasn't clear to me, there is some explanation here: http://www.lua.org/pil/27.3.3.html http://www.lua.org/pil/27.3.3.html Essentially, they are unbound variables that have to be reachable from the function by some mechanism, making it a closure. Apologies if I've misunderstood anything here.
- zeen 15y ago
  You got it. Quoting from the 5.2 manual: Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function. i.e., the external local variables the closure is closed on. And for C functions made available to Lua scripts: When a C function is created, it is possible to associate some values with it, thus creating a C closure (see lua_pushcclosure); these values are called upvalues and are accessible to the function whenever it is called. - http://www.lua.org/manual/5.2/manual.html http://www.lua.org/manual/5.2/manual.html
carterschonwald 15y ago
What this leaves me wondering is : what are the continuation semantics for the gotos? Without that part it's hard to know if "ill formed" gotos that result in trying to execute expressions with undefined variables can happen or not.
- justincormack 15y ago
  A label is visible in the entire block where it is defined (including nested blocks, but not nested functions). A goto may jump to any visible label as long as it does not enter into the scope of a local variable. This can be determined statically.
  - carterschonwald 15y ago
    ok, so the semantics for lua goto's is to enter at the start of smallest enclosing block that has no free variables in the current environment? Or to statically reject code that tries to do so? That doesn't sound quite accurate, unless you're saying that lua does some static analysis before execution, which I was not aware of.
    - Dylan16807 15y ago
      Lua compiles to a simple bytecode. It makes sure gotos are valid during this compilation. It jumps directly to the label, and can't skip forward over the declarations of variables that are in scope at the label.
      - carterschonwald 15y ago
        cool. Thanks for the info. Where are this and other features nicely documented?
        Dylan16807 15y ago
        I would suggest reading The Implementation of Lua 5.0 http://www.lua.org/doc/jucs05.pdf http://www.lua.org/doc/jucs05.pdf
andrewcooke 15y ago
can someone smarter than me comment on whether delimited continuations are better or worse than goto and/or "ordinary" continuations? (because i suspect they are nicer in how they handle scope, and they are what are typically implemented, aren't they?)
zura 15y ago
I think it is worth to mention: Structured programming with go to statements (1974) by Donald E. Knuth (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.6084 http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.6...)

35 points by telemachos 15y ago

The attitude towards goto really struck a chord with me. It's "powerful" and "easy to explain". It's also "simple" to add to the VM. It's in, and purity be damned. There's something to be said here about pragmatism versus purity. Maybe also a comparison with the recent posts about OO and Rails. I'm hand-waving. I mean to write something longer about this, but I'm still trying to decide what exactly I make of it.

justincormack 15y ago
A lot of people wanted it is Lua is quite widely used as a code generation target, and generating gotos is useful often to match other language constructs.

watmough 15y ago

For anyone wondering about "C upvalues", which wasn't clear to me, there is some explanation here: http://www.lua.org/pil/27.3.3.html http://www.lua.org/pil/27.3.3.html Essentially, they are unbound variables that have to be reachable from the function by some mechanism, making it a closure. Apologies if I've misunderstood anything here.

zeen 15y ago
You got it. Quoting from the 5.2 manual: Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function. i.e., the external local variables the closure is closed on. And for C functions made available to Lua scripts: When a C function is created, it is possible to associate some values with it, thus creating a C closure (see lua_pushcclosure); these values are called upvalues and are accessible to the function whenever it is called. - http://www.lua.org/manual/5.2/manual.html http://www.lua.org/manual/5.2/manual.html

carterschonwald 15y ago

What this leaves me wondering is : what are the continuation semantics for the gotos? Without that part it's hard to know if "ill formed" gotos that result in trying to execute expressions with undefined variables can happen or not.

justincormack 15y ago
A label is visible in the entire block where it is defined (including nested blocks, but not nested functions). A goto may jump to any visible label as long as it does not enter into the scope of a local variable. This can be determined statically.
- carterschonwald 15y ago
  ok, so the semantics for lua goto's is to enter at the start of smallest enclosing block that has no free variables in the current environment? Or to statically reject code that tries to do so? That doesn't sound quite accurate, unless you're saying that lua does some static analysis before execution, which I was not aware of.
  - Dylan16807 15y ago
    Lua compiles to a simple bytecode. It makes sure gotos are valid during this compilation. It jumps directly to the label, and can't skip forward over the declarations of variables that are in scope at the label.
    - carterschonwald 15y ago
      cool. Thanks for the info. Where are this and other features nicely documented?
      - Dylan16807 15y ago
        I would suggest reading The Implementation of Lua 5.0 http://www.lua.org/doc/jucs05.pdf http://www.lua.org/doc/jucs05.pdf

andrewcooke 15y ago

can someone smarter than me comment on whether delimited continuations are better or worse than goto and/or "ordinary" continuations? (because i suspect they are nicer in how they handle scope, and they are what are typically implemented, aren't they?)

zura 15y ago

I think it is worth to mention: Structured programming with go to statements (1974) by Donald E. Knuth (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.6084 http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.6...)