Roll A Lisp In C – Reading (2020)

https://swatson555.github.io/posts/2020-01-18-make-a-lisp-1.html

by swatson741 • 2 years ago

65 21 2 years ago

swatson555.github.io

okl• 2 years ago

Tokenizer is unsafe because the loops are not bounded to the array sizes, 128 and 32 respectively.

vendiddy• 2 years ago

Anyone have a good reference on how write the interpreter itself?

I'm looking to write a really basic one in javascript except using plain arrays. No parsing or tokenizing for me.

mtreis86• 2 years ago

Many are based on the SICP metacircular evaluator. Write that in your language of choice then you can bootstrap into scheme from there. Also see the 1986 MIT lecture videos by the authors, to get the spirit of the thing.

mikelevins• 2 years ago

The first time I attempted a Lisp back in about 1989 or something, I used the metacircular evaluator from SICP.

A little later I used Norvig's Scheme implementation from chapters 22 and 23 of PAIP (https://norvig.github.io/paip-lisp/#/?id=paradigms-of-artifi...).

I liked Norvig's a little better. It starts with a simple interpreter and adds improvements step-by-step, winding up with an optimizing compiler that targets a bytecode VM.

That approach makes a lot of implementation details manifest in a way that is easy to grasp. For example, I think I understood continuations much better after seeing Norvig's account of how to implement them.

ggliv• 2 years ago

While probably a bit overkill for writing a Lisp, Robert Nystrom’s Crafting Interpreters [0] is a really fantastic book on the topic.

[0] https://craftinginterpreters.com/contents.html

Zambyte• 2 years ago

https://sicp.sourceacademy.org/chapters/4.html

This inlines the diffs between the original SICP and the new one that is implemented in Javascript too.

jetti3• 2 years ago

I’m not really sure how you would be able to write an interpreter without a parser, since you need to know what the user is trying to do. For a more complete tutorial there is Make-A-Lisp (mal) [0] that has steps for making a lisp in various (including JavaScript) and has a process guide [1] to get started.

[0] https://github.com/kanaka/mal

[1] https://github.com/kanaka/mal/blob/master/process/guide.md

KineticLensman• 2 years ago

I can second Make-A-Lisp. I worked through it in C# initially, and I'm now halfway through a Rust implementation. The MAL process guide is a good set of instructions but doesn't go as far as actually presenting pseudocode, so you do have to think, and sometimes 'cheat' by looking at one of the reference implementations. In terms of tokenising and parsing, the MAL guide does include a monster regex statement that recognises the relevant tokens, and the parsing is quite easy due to Lisp's relatively simple syntax. If you choose the right implementation language, it will do some of the heavy lifting for you (notably memory management).

[Edit] Just wanted to reiterate that lexing/parsing Lisp is easy as per the MAL instructions. IIRC the things that I found hardest (in C#, as a C# newby) were understanding how to implement the closures required to support function definition, and then implementing Lisp's macro expansion.

naasking4• 2 years ago

You can create an interpreter in the finally tagless style:

https://okmij.org/ftp/tagless-final/

This embeds the target language as combinators in a host language, so you can construct and evaluate programs.

fmbb• 2 years ago

Check out miniMAL from the person behind Make a Lisp. It’s exactly what you want, I think: http://kanaka.github.io/miniMAL/

im3w1l• 2 years ago

Here's some pseudocode that should help you get started on a very simple thing

  evalExpression:
    switch peekChar
      ( evalFunctionCall
      0-9 parseNumber
      " parseString
      a-zA-Z parseAndGetVariable

  evalFunctionCall
     fn = evalExpression
     arguments = evalExpression  until )
     fn(arguments)

matheusmoreira• 2 years ago

Seconding the Make-A-Lisp and Crafting Interpreters recommendations. TFA is also pretty good, it contains a hard written parser that you may learn from. Lisp grammar is regarded as simple enough that full parser generators need not be used.

There are also the Lis.py articles which are incredibly good:

https://norvig.com/lispy.html

https://norvig.com/lispy2.html

You mentioned you want to write your lisp in javascript so you'll be getting quite a lot of functionality for free: memory allocation and garbage collection. That alone solves an enormous amount of problems. Just in case anyone reading is going to write in C, here's an excellent introduction to the topic:

https://journal.stuffwithstuff.com/2013/12/08/babys-first-ga...

Don't forget to spill the registers.

anonzzzies2• 2 years ago

I have been using sbcl for a very long time and never knew that the core is a small optimised c part and the rest all written in Lisp. Not totally self hosted but quite close.

Now I have been searching for an overview of the CL commands that make up that core which would be needed to implement CL (run the rest), but I cannot find any. Anyone here knows?

p_l2• 2 years ago

The C runtime is pretty much all OS-interaction support components.

IIRC, none of the actual ANSI CL code is implemented in C - in fact, it would break one of the core design ideas of SBCL, which is "Sanely Bootstrappable" - SBCL can compile itself on any "complete enough" ANSI CL system - it doesn't even have to implement everything - then uses itself and its own standard library to compile itself again but into actual native code.

Out of memory, the C based runtime in SBCL has the garbage collector, low-level memory interfaces to OS (how to acquire memory, how to release memory, setting memory protection flags), some core FFI to OS which are called pretty deep from standard library (i/o, threads, processes), basic FFI supports (manipulating linkage tables, for example) setting CPU contexts, etc.

On the non-OS-specific side, there's core loading, which is a distant ancestor of Mach dynamic linker.

Some of it is C-side implementation of basic SBCL datatypes which allows both GC and runtime to interact with them safely, but usually the CL code does not call into those.

Some of the low-level manipulation is actually assembler.

anonzzzies2• 2 years ago

Thanks, that’s very helpful! I wish people would write detailed implementation books about these things as it is very hard to get into when just jumping in the deep even for experienced C and Lisp programmers. I guess I need to jump in!

Roll A Lisp In C – Reading (2020)

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