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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 o

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 2y 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 2y 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-artificial-intelligence-programming 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 2y 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 https://craftinginterpreters.com/contents.html
Zambyte 2y ago
https://sicp.sourceacademy.org/chapters/4.html 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.
jetti 2y 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 https://github.com/kanaka/mal [1] https://github.com/kanaka/mal/blob/master/process/guide.md https://github.com/kanaka/mal/blob/master/process/guide.md
- KineticLensman 2y 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.
- naasking 2y ago
  You can create an interpreter in the finally tagless style: https://okmij.org/ftp/tagless-final/ https://okmij.org/ftp/tagless-final/ This embeds the target language as combinators in a host language, so you can construct and evaluate programs.
  - jetti 2y ago
    I have not heard of tagless final style interpreter before and that seems really neat. However, based on the link it seems that tagless final interpreter uses a strong type language like Haskell or ML so I don’t know how well it would translate to Javascript, which is what the GP comment said they were using.
fmbb 2y ago
Check out miniMAL from the person behind Make a Lisp. It’s exactly what you want, I think: http://kanaka.github.io/miniMAL/ http://kanaka.github.io/miniMAL/
im3w1l 2y 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 2y 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/lispy.html https://norvig.com/lispy2.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-garbage-collector/ https://journal.stuffwithstuff.com/2013/12/08/babys-first-ga... Don't forget to spill the registers.

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 2y 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 2y 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-artificial-intelligence-programming 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 2y 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 https://craftinginterpreters.com/contents.html

Zambyte 2y ago

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

jetti 2y 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 https://github.com/kanaka/mal [1] https://github.com/kanaka/mal/blob/master/process/guide.md https://github.com/kanaka/mal/blob/master/process/guide.md

KineticLensman 2y 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.
naasking 2y ago
You can create an interpreter in the finally tagless style: https://okmij.org/ftp/tagless-final/ https://okmij.org/ftp/tagless-final/ This embeds the target language as combinators in a host language, so you can construct and evaluate programs.
- jetti 2y ago
  I have not heard of tagless final style interpreter before and that seems really neat. However, based on the link it seems that tagless final interpreter uses a strong type language like Haskell or ML so I don’t know how well it would translate to Javascript, which is what the GP comment said they were using.

fmbb 2y ago

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

im3w1l 2y 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 2y 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/lispy.html https://norvig.com/lispy2.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-garbage-collector/ https://journal.stuffwithstuff.com/2013/12/08/babys-first-ga... Don't forget to spill the registers.