5 ms·

I feel like writing a LISP in a gc'ed language is taking away most of the fun of it. I wrote one in C (without a conservative garbage collector) and the most i

by rrmm 9y ago

I feel like writing a LISP in a gc'ed language is taking away most of the fun of it. I wrote one in C (without a conservative garbage collector) and the most interesting aspect was what had to be done to allow garbage collection. Garbage collection can run inside nearly all subroutines. So any temporary references stored outside of the heap need to be kept track of at all times. If gc occurs you have to able to walk the entire heap and the entire callstack to mark live data. This is obvious but I enjoyed finding it out (and subsequently rewriting everything).

DonaldFisk 9y ago
> I feel like writing a LISP in a gc'ed language is taking away most of the fun of it. Try telling Rich Hickey that. There's also Kawa and Armed Bear Common Lisp running on the JVM. If you write your interpreter in a non-garbage-collected language (e.g. C), you should use an explicitly declared stack for arguments, and have argument-free functions. Also, for functions which might call the garbage collector, you can't store any garbage collectable data in local variables either. It might be possible to have your garbage collector access the C call stack, but I wouldn't recommend that. Three decades ago, my first attempt at implementing a Lisp interpreter in Pascal fell foul of this problem and occasionally recycled lists which were still in use. I soon figured out what was happening. I decided to replace the interpreter with a virtual machine and a compiler (and also a source-code interpreter for bootstrapping and the REPL), and I still use a descendent of this.
munificent 9y ago
One of my seminal experiences in hacking on languages was working on a VM for a garbage collected language written in C++. I wrote a Cheney style copying collector to avoid fragmentation. Meanwhile, I used C++ inheritance and vtables to implement the different kinds of objects supported by the runtime. I thought it was a really cool system until I realized that the GC could move the object pointed to by `this` in the middle of a method. Ouch. :(
marcpaq 9y ago
The irony of implementing Lisp in assembly is that, almost for free (sorry about that), you get precise, convenient control of the memory situation. Just put the gc root exclusively in registers. I wrote a Lisp interpreter in assembly that dedicates 4-5 registers for the current expression, eval environment, and so on. The mark-sweep collector starts from these registers. https://github.com/marcpaq/arpilisp https://github.com/marcpaq/arpilisp
- rrmm 9y ago
  Cool project. It's much clearer than I would have thought in asm. The inline documentation is good.
- kazinator 9y ago
  No irony there. You need precise control over the machine to implement a paradigm accurately from the ground up. It's hard to do that using someone else's paradigm, living with their choices of what aspects of the machine are still revealed and which are hidden.

by rrmm 9y ago

DonaldFisk 9y ago

> I feel like writing a LISP in a gc'ed language is taking away most of the fun of it. Try telling Rich Hickey that. There's also Kawa and Armed Bear Common Lisp running on the JVM. If you write your interpreter in a non-garbage-collected language (e.g. C), you should use an explicitly declared stack for arguments, and have argument-free functions. Also, for functions which might call the garbage collector, you can't store any garbage collectable data in local variables either. It might be possible to have your garbage collector access the C call stack, but I wouldn't recommend that. Three decades ago, my first attempt at implementing a Lisp interpreter in Pascal fell foul of this problem and occasionally recycled lists which were still in use. I soon figured out what was happening. I decided to replace the interpreter with a virtual machine and a compiler (and also a source-code interpreter for bootstrapping and the REPL), and I still use a descendent of this.

munificent 9y ago

One of my seminal experiences in hacking on languages was working on a VM for a garbage collected language written in C++. I wrote a Cheney style copying collector to avoid fragmentation. Meanwhile, I used C++ inheritance and vtables to implement the different kinds of objects supported by the runtime. I thought it was a really cool system until I realized that the GC could move the object pointed to by `this` in the middle of a method. Ouch. :(

marcpaq 9y ago

The irony of implementing Lisp in assembly is that, almost for free (sorry about that), you get precise, convenient control of the memory situation. Just put the gc root exclusively in registers. I wrote a Lisp interpreter in assembly that dedicates 4-5 registers for the current expression, eval environment, and so on. The mark-sweep collector starts from these registers. https://github.com/marcpaq/arpilisp https://github.com/marcpaq/arpilisp

rrmm 9y ago
Cool project. It's much clearer than I would have thought in asm. The inline documentation is good.
kazinator 9y ago
No irony there. You need precise control over the machine to implement a paradigm accurately from the ground up. It's hard to do that using someone else's paradigm, living with their choices of what aspects of the machine are still revealed and which are hidden.