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Mal – Make a Lisp, in 68 languages (github.com)
236 points by ingve on Sept 12, 2017 | hide | past | web | favorite | 69 comments

I got decently far in my implementation, but probably wouldn't suggest you use this. The actual guide is pretty sloppily written and doesn't outline all that much, along with not explicitly saying what you have to define at each step. It doesn't define what the different cases of the mal Lisp value type should be, for example, or when to implement pretty much all of the "optional" extensions it mentions in the first or second chapter. Most of the forms in each chapter get a sentence explanation of what they do, at most, and I hit several snags where I read an explanation and thought I implemented it correctly, only to find out there was something subtly wrong with the obvious implementation.

Also, writing a Lisp in a purely functional language isn't the best :X Things like by-reference function environments and atoms are hard to implement if you're not Haskell, at least if you try in the method that the guide outlines...

There is a similar project called "Build Your Own Lisp"[0] (BYOL), which is a book that walks one through making a Lisp in C. Has anybody here done BYOL? How does it compare to MAL? If you had to choose one to learn how to create a Lisp, which would you choose?

[0]: http://www.buildyourownlisp.com/

BYOL is targeted at implementing Lisp in C and focuses on learning C. There is a fair bit more hand-holding and example code in C for most of the steps than in the mal guide. BYOL is more polished and written in the form of a short book whereas the mal guide intentionally tries to be more concise and informal in style. I would read through the first couple of sections of both and decide based on your own goals and preferences.

Actually you might like stage0 http://git.savannah.nongnu.org/cgit/stage0.git/ It literally goes from a 280byte hex monitor all the way through a compacting garbage collected lisp and FORTH

Without the assumption that any other software exists

I used build your own lisp to learn C (along with other stuff) I liked it, except I dont think its the best way to learn to develop languages.

I looked at BYOL a while ago, but was turned off by the fact that the author wants you to use his parser. I don't know why, but I didn't like that. I feel like there's value in using a pre-existing, widely-used parser like yacc/lex, for no other reason than that it already exist and is widely used.

Oh man, that's a bummer. I think one of the great joys in constructing your own compiler or interpreter is making your own lexer and parser. Doesn't using a pre-made lexer/parser kind of defeat the purpose of making your own compiler/interpreter?

This is a fantastic way to "waste" a weekend, and there's so much learning to be had.

The radically simple implementation of TCO really surprised me.

See the design narrative at “Step 5: Tail call optimization”[0] in The Make-A-Lisp Process[1].

[0]: https://github.com/kanaka/mal/blob/master/process/guide.md#s...

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

Writing/understanding a lisp interpreter is a longstanding item on my TODO list.

Question: how do I go from tackling this to tackling compilers?

Make it generatem a simple bytecode format, that you can map into Assembler macros.

Choose a good Macro Assembler like NASM or YASM, and implement the bytecodes as macros.

It won't win any benchmark, but you will get native executables.

Or use Lisp macros to generate the code.

Here is a Scheme based tutorial.


Make a very decent interpreter for a language that is good for writing compilers (without a question, a Lisp of some sorts, of course, ha).

Then, write the compiler in that language.

The compiler walks the code just like the interpreter, but instead of interpreting, it spits out a translation.

Spitting out some kind of translation into something isn't a very high bar; the difficulties are all in optimization: things like analyzing lexical scopes for the presence of closures which escape, so as to be able to stack-allocate environments that aren't captured by closures. Plus all those other difficult things: like turning a program into a graph of basic blocks and doing flow analysis; register allocation, selection of instructions for a myriad target machines, debugging suport, and so on. Any of these requirements can be excluded from a compiler, yet it's still a compiler.

The "Lisp in 500 lines of C" (plus accopmanying .lisp) file implemented a compiler. The complete source is hard to find, but here is a derivative project: https://github.com/jackpal/lisp5000

A chunk of init500.lisp implements compilation to C. Search the file for terms like "compile" and also "deftransform".

If you've ever written a macro, you've written a piece of a compiler. Macros take an input syntax tree and spit out a new one. Many actions traditionally associated with a compiler (by Lisp-unaware computer scientists) can be done by macros or in the macro expander. Basic compiling is a lot like macro expansion. It can actually be Lisp-to-Lisp even; the transformers can spit out a Lisp data structure representing a target language. (Heck GCC spits out a Lisp-like data structure for intermediate code. Of course; it was originally a project started by Stallman.)

Work through The Make-A-Lisp Process[0], which finishes at “Step A: Metadata, Self-hosting and Interop.”[1]

From there, you’ll be more than ready to generate machine code.

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

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

I was just recommended "Lisp in Small Pieces" for just this.

I have that book, though I haven't looked at it in years. I found it intractable then. I've been using clojure in the interim and feel like I've learned quite a bit, so maybe I should go back and look again.

Also check out miniMAL [0] - lisp in JSON form!

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

Could someone explain this gem in the C version?


In struct MalVal[0], the values f0 through f20, where each holds a pointer-to-functions of the corresponding arity, live in a big (multifacted?) union. Using arg_cnt, the switch assigns func to the appropriate value in mv->val using the appropriate cast.

For example, when arg_cnt is 2, mv->val.f2 receives func converted to void * (* )(void* , void* ), that is, pointer to a function that accepts two parameters of type pointer-to-void and returns pointer-to-void. [Weird spacing to effectively escape the asterisks that would otherwise be treated as italicizing markup.]

C permits conversion between pointer-to-function of one type to pointer-to-function of another type. The inverse is also defined, and the resulting pointer will compare equal to the original. However, calling a function through a pointer-to-function where the types do not match invokes undefined behavior.

[0]: https://github.com/kanaka/mal/blob/master/c/types.h#L85

This could be made massively less ugly and more readable by typedef'ing the function pointer types by the way. Then one could just write:

  case 13: mv->val.f13 = (F13)func; break;
As a C programmer I got to say people in general don't use typedef for function pointer types often enough. The types are so ugly and verbose, they should be typedef'ed by default.

I think the primary argument that I've heard against typedefing function pointers is you can end up obfuscating what is happening and end up making the code less readable/harder to maintain in the future.

This code could take advantage of the fact that the type is a union.

One can bend the rules of prim-and-proper well-defined ISO C just a little bit and assign to the simplest function pointer, taking advantage of all function pointers having the same representation on every machine known to mortal hacker, and all being overlaid by the union. Thus all the cases collapse down to this:

   mv->val.u.f0 = (void (*)(void)) func;
done. Now if this is 13 arguments, then strictly speaking, accessing mv->val.uf13 is not well-defined behavior. That's only going to be problem when it actually doesn't work, though. A big problem, then. :)

I can respect the author taking the trouble to avoid undefined behavior. Technically correct is the best kind of correct.

That sort of love affair with ISO gospel lands on the rocks as soon as your Lisp stuffs a couple of tag bits into a C pointer.

I look forwards to your pull request! One challenge you'll run into is that (as I recall) the Boehm GC linkage no longer works with newer versions of Boehm.

In TXR Lisp, I did the grunt work and wrote separate constructor functions for different function signatures:


These functions are only used from within C code; they are not the basis for any intrinsic functions. The call to each of these functions knows exactly the type of function it wants to make.

There is a notation in the naming.

Firstly, the leading 'f' denotes functions with an environment value, whereas 'n' denotes non-environment functions. Then there is a number indicating the number of arguments. Then an optional letter which is 'o' if the function has optional arguments, or 'v' if it is variadic. Both can be present in which case we have 'ov.

Thus for example func_n2ov(some_c_function, 1) will create a variadic function with 2 fixed arguments, of which 1 is required (so one optional). some_c_function has to have the type signature val (* )(val, val, varg). No casting is required because func_n2ov just assigns this to the correct union member. If we pass a function with an incorrect signature, we get a C compile error.

The func_n2ov constructor is used when registering the unique function in eval.c:

  reg_fun(intern(lit("unique"), user_package), func_n2ov(unique, 1));
We intern the string "unique" in the user package ("usr"), and use that symbol to register a function createdy by hoisting the C function unique into the Lisp domain with func_n2ov.

I wrote TXR in a way that is easy to understand and maintain. It is only implemented in C, though. If you want to study how to make a Lisp interpreter in C. Yet, it is production code for real work.

There is another Lisp implemented in 42 languages (including Ceylon, Dylan, Oz, Pike, Scratch, Smalltalk, SML etc.):


The benchmark results of 42 implementations are very impressive:

http://blog.bugyo.tk/lyrical/wp-content/uploads/2014/12/stag... http://blog.bugyo.tk/lyrical/wp-content/uploads/2014/12/stag... http://blog.bugyo.tk/lyrical/wp-content/uploads/2014/12/stag...

which are discussed in Japanese at:


Not to toot my own horn too much, but I have also written a tutorial[0] aimed at building a Lisp in OCaml. I wrote it to teach as much as learn how to go about it.


I am curious why is hackernews always so happy and obsessed with lisp, there is almost daily a lisp post on the frontpage. I dont really care/mind but I am just curious as why people love lisp so much. The only thing that I notice about Lisp are the bracket jokes and memes.

Paul Graham and Robert Morris built one of the first web applications (that's a PG claim, btw) that allowed users to build their own, separate websites that had shopping carts, custom UI, etc.

They wrote the app in Common Lisp and claimed that it was their secret weapon which allowed them to iterate faster and stay ahead of their competition. They ended up selling their company (ViaWeb) to Yahoo for $50 million (IIRC).

PG also authored two books on Lisp, and HN is written in Arc (a Lisp built on top of Racket).

Around 2005 I was doing a lot of Lisp programming and I was really interested in what PG had to say. He was writing a lot of essays then and giving lots of talks, etc.

Unfortunately, I don't get the opportunity to use Lisp as much as I used to, but I'm doing a lot of Python and even played around with Hy (Lisp on top of Python). So I'm interested in Lisp as I find it to be an incredibly powerful language that is really fun to use.

After being a user of languages for a long time, writing my own LISP was the experience of leaving Platons cave and seeing what the thing I use daily really is made of. What is "if"? What is a boolean, a variable, a function?

It's a beautiful thing. I wish I would never have to climb down into the cave (languages with more advanced parsers) again.

You climed in the wrong direction. What you want is assembly. That's where software ends up.

Also, it's interpreters all the way down. Even assembly is interpreted.

Which might be a Scheme CPU formally derived and verified via LISP-based tech:


Or recently this way:


Interesting enough, the primitive versions of LISP that aren't high-performance (i.e. advanced compilers) could probably be done by hand in hardware where the whole thing was bootstrapped up with LISP-only tech. The LISP would start/stop at the abstract, state machines or RTL of the bootstrapped version.

EDIT: Good luck on making it through the hurricane. Feel for yall out there.

Not the wrong direction, just not far enough.

Learning assembly is great, and every programmer should dive into at least a simpler version of it, but it's not exactly the best way to learn how your high-level language is implementing tail-call optimization.

>What you want is assembly.

Which you can code in a convenient way by using Lisp and Lisp macros to generate the <insert target CPU> machine language output using <insert target CPU> opcodes.

>Also, it's interpreters all the way down. Even assembly is interpreted.

Speak truth to power. Every time someone says this there seem to be nothing but haters.

> Every time someone says this there seem to be nothing but haters.

Because it's not completely true. If a CPU is microcoded, then it's accurate to say "assembly is interpreted" because every instruction is effectively an address into a lookup table of microinstructions. But in a non-microcoded (e.g. purely RISC) CPU, the bits of the instruction word are effectively enable and data lines to a bunch of hardware logic gates and flip-flops, which cause register transfers and arithmetic operations to happen. In this case, the ones and zeros in the instruction word are voltage levels on logic gates. Calling the latter "interpretation" is a stretch.

To be fair, there aren't many pure RISC implementations around these days. Most everything has some degree of microcode involved, so to that extent you're right.

It's interpreted because the instructions are fetched one by one. A piano roll is intepreted, even though its holes just activate keys with a "horizontal encoding". It is interpreted because it moves through the piano, and a little piece of it activates a behavior any one time, without leaving a permanent record.

Not only is machine code interpreted, the so-called "asynchronous interrupts" are just periodically polled for in between the fetching, decoding and executing.

>Even assembly is interpreted.

Could you please elaborate on this point? I'm still very much in the cave.

I'll use x86-32 for elaboration [1]. When the CPU sees the byte sequence 0xB8 0x90 0x41 0x5A 0x7B, it has to interpet what those bytes mean. It sees the 0xB8, so then it knows that you are loading the EAX register with an immedate value. The next four bytes (0x90, 0x41, 0x5A, 0x7B) are read and stored into EAX (as 0x7B5A4190, because Intel is little endian).

That is the case for all instructions. Each one is interpreted by the CPU. And for modern CPUs, even translated into an internal format that is futher interpreted.

[1] Sans power right now. Using my cellphone as a hot-spot and my iPad as a laptop. The aftermath of hurricanes is brutal in Florida.

Gotcha, but then (unless I'm misunderstanding) one of these interpreters is not like the other. Namely, assembly 'interpretation' happens on bare metal. Were you previously suggesting that understanding a lisp interpreter will help in understanding CPU architecture?

(Good luck recovering from the hurricane! Keep your head down!)

I was replying more to jospar's post about learning what an 'if' was, what a 'boolean' was, etc. What's an 'if'? Ultimately it's a comparison of two numbers and a transfer of control based upon said comparison. Some architectures that's one instruction, some two.

Lisps have properties that are not so common in other languages:

- Homoiconicity: Code and data are the same (s-expressions)

- Lisp code is basically the AST in itself

- It's trivial to implement lisp in lisp (eval)

- Continuations (call/cc)

- Macros

- etc...

It's a truly fascinating language. I never really did any Lisp coding outside some university projects and I still obsess and read about Scheme all the time.

>I am just curious as why people love lisp so much.

Lisp is addictive, like a hard drug.

Lisp is the original "programmable programming language".

Experienced programmers with no previous knowledge of Lisp should relish (or doubt in disbelief) at the features provided.

Lisp, at least Common Lisp (but other Lisp dialects as well) might claim to be the most powerful and versatile programming language (and environment) available.

CL can run pretty fast. At the same speed than Java on the oracle JVM, and if some tricks are applied it should match C and Fortran speed under certain conditions. CL code is extremely portable.

Lisp has been used for very high level tasks (AI, etc) and for low level tasks (operating systems for Lisp machines).

Lisp is one of those languages that, once I learned it (Clojure in my case), I started wondering why every language wasn't more like it. Once you get used to the parentheses and whatnot, it's actually an incredibly beautiful language that gives you the ability to augment the language without waiting for updates via its elegant macro system.

> The only thing that I notice about Lisp are the bracket jokes and memes.

You have also noticed that lots of people use it to create cool things which reach the HN front page.

Including the front page itself.

Thanks for all the explanations. Now I understand why u guys get excited from it. Almost sounds to good to be true.

Because lisps are unique and cool. Really, they are quite special.

I tried finding the lines of code in each language with cloc. C/C++ Header contains a sum of C and CPP header files. Python has been used across different language dirs as wrapper script or so, otherwise, it's own implementation is probably 1350 lines at max. This would also probably include bugs in cloc.

    github.com/AlDanial/cloc v 1.70  T=2.86 s (433.6 files/s, 67570.0 lines/s)
    Language                      files          blank        comment           code
    Visual Basic                     36           1569            132           8036
    Swift                            36           1199           1606           7951
    SQL                              36            956            649           7594
    Pascal                           19            704            951           6253
    Ada                              28           1975            380           5856
    Elm                              19           1619            226           5647
    awk                              16            290             15           5169
    Lisp                             37            825            256           4562
    VHDL                             17            434             32           4228
    C#                               20            658            346           4175
    Python                           37            574            394           4018
    make                             89           1244            873           4010
    C                                19            466            392           3499
    Perl                             35            413            171           3450
    Go                               17            281            148           3397
    Rust                             18            304            130           3331
    JavaScript                       43            449            247           3235
    Forth                            18            516            158           3228
    C++                              18            568             71           2986
    Java                             17            325            135           2938
    D                                17            363             10           2938
    Racket                           35            480            385           2740
    TypeScript                       17            284             56           2740
    Markdown                          8            714              0           2730
    Rexx                             17            283             47           2713
    Bourne Shell                     30            390            315           2692
    Tcl/Tk                           17            284             43           2416
    Dart                             16            255             41           2303
    F#                               20            337             13           2298
    Objective C                      17            285            150           2189
    Erlang                           17            277            191           2185
    MATLAB                           26            184            128           2165
    Haxe                             19            254             79           2161
    Crystal                          18            417             58           2146
    vim script                       17            242             50           2076
    Haskell                          17            331             99           2000
    PHP                              18            249            120           1955
    Lua                              18            248             87           1874
    Scala                            16            221            113           1816
    Elixir                           19            366             35           1809
    JSON                             28            246              0           1765
    R                                17            201            100           1612
    Groovy                           17            170             98           1582
    Kotlin                           17            312              0           1554
    Julia                            17            189            126           1405
    Nim                              16            320             36           1397
    lex                              18            209              0           1355
    Ruby                             17            167             87           1264
    OCaml                            15            109             98           1211
    PowerShell                       11            128             71           1146
    ClojureC                         15            257            129           1053
    CoffeeScript                     17            195            126           1037
    CSS                               6            150            132            792
    C/C++ Header                     22            266             44            752
    HTML                              2             36             35            486
    Bourne Again Shell               47              1              4            135
    YAML                              2              6             10             86
    Maven                             1              2              7             85
    Clojure                           2              8             12             65
    ClojureScript                     1              1              0              2
    SUM:                           1242          24806          10447         158293

Every implementation has a stats target that gives byte counts, LOCs, and comments that is specific to that implementation. I.e. from the top-level you can run the following to get stats for every language:

  make stats
or for more condensed form:

  make stats | egrep "Running|total|comments"

Note that the number of intermediate "steps" uploaded to github, as well as the number of tests, differ for each language.

The steps and main files are the same for every implementation (that's part of the requirements for merging into the main tree). Some implementations have additional files like readline, utility routines, etc. But for the most part the general structure and file divisions are very similar.

What is the smallest Mal implementation? OCalm?

Implementation size is hard to compare accurately across languages (bytes? lines of code? exclude comments? excluding leading spaces?). Also, the mal implementations were created by many different people so individual style plays a large role in concision/verbosity.

However, that being said, the following implementations are "smallish" (in both lines of code and bytes): mal itself (self-hosted), Clojure, factor, Perl 6, Ruby. The following are "largish": Awk, PL-pgSQL, C, PL-SQL, Chuck, Swift 2, Ada.

OCaml is in the "smallest" third of the implementations.


It started with the question "Could you implement a Lisp using just GNU Make macros"? (Hint: Yes) Bit of trivia: Mal originally stood for MAke-Lisp. Conveniently the acronym didn't need to be changed for "Make-A-Lisp"

It then grew into a personal learning tool for me for learning new languages. As I implemented more languages, I refactored the structure to make it more portable and incremental. At some point I realized that it might be an interesting learning tool for others so I wrote an early version of the guide and had a friend work through it and give feedback. He liked it enough that he immediately did a second implementation.

At some point the project got some twitter/HN attention and other people began contributing implementations and feedback for the guide.

Mal also serves as programming language "Rosetta Stone". A bit like rosettacode.org but using a full working project.

Writing an interpreter (or compiler) greatly enhances one's programming skills.

Why not work on improving an existing one ?

While we're at it, why don't we replace high school math tests with one question: "Solve the Riemann hypothesis". We can replace the physics curriculum with "Unify quantum mechanics with general relativity", and throw out computing courses in favour of "Prove whether or not P = NP".

It's important to re-do things which others have already worked out, in order to reach a greater understanding :)


I learned how to implement Lisp by reading the source code for multiple compilers. Started with Franz Lisp, then KCL and CMUCL. I don't feel that a toy interpreter is going to teach the same things.

Perhaps you could write up the process you followed, in the form of blog post or something, so that you can pass on the knowledge. I, for one, would love a guided tour through those implementations to see which parts are worth studying to understand how an established Lisp is built.

everyone learns in different ways, don't be too attached to your own perspective. we all interpret the world through different lenses and need to learn through the expressions of different methodologies. some people benefit heavily by implementing something themselves and not just reading through something someone else wrote. some people feed off of that practical and robust experience of implementation in order to get a proper context for how things actually work established in their head. concrete examples are more helpful than you might think to people who aren't as capable of abstract thought and logical analysis of something from its constituent parts

Maybe part of my reaction to this is a wish to avoid reinforcing the "Lisp is an interpreted language" meme.

I don't have the time to learn it that way. It's this or never doing it at all.

Your approach is 100% valid, but it can't hurt to start making a really small lisp. Of course the implementation strategies of a "MAL" lisp and a Real Lisp are entirely different.

Other people have different preferred learning styles.

A toy language won't teach the exact same things, but it covers a lot of the same topics.

In order to learn and build up a base of skills.

Edification / amusement.

I had a go in Rust recently. Was a great way to learn new things about Rust and also some interpreter concepts.

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