My experience of writing Lisp in Pony

Part of "Make-A-Lisp in Pony" series

  1. Make a Lisp - review
  2. My experience of writing Lisp in Pony

This is the second part of my reflection on implementing Lisp in Pony (following Make-A-Lisp tutorial).

I don’t have previous experience in Pony. This is an impression of the newcomer. Also keep in mind, that Pony is still being actively developed. They have ambitious ideas, but they are not quite there yet.

What I like

I like that Pony doesn’t afraid to break some programming language “traditions”, which leads to cleaner language. (Example of “tradition”: CAR, CDR and CONS in Lisp.)

To get a value from an array you would use parenthesis, so it looks like a function call:


If you think about it, it is indeed a function call you “map” from index to the value in that cell of the array. And this function is partial (marked with ?), which means that not for every input you will get an output. In contrast, many languages will silently ignore this error and return null (or nil, or none - whatever you call it).

It has nice and concise syntax. It looks like Python, but it isn’t indentation sensitive.

It has a pretty big standard library, I didn’t get to use a lot of it (because you don’t need that many things to build Lisp).

There are union types and “symbols”, like in many functional programming languages (I like those) and you can do pattern matching on types.

Overall language works as you expect.

What was hard


Reference capabilities (refcap) is the hardest part. But it is expected, there is the same experience with Rust borrow checker. I didn’t get a full grasp of the refcap - I rather learned to workaround (just use ref everywhere).

I watched a couple of videos about it and it seemed I understood the idea (it makes sense):

Deny global read/write aliasesDeny global write aliasesDon’t deny any global aliases
Deny local read/write aliasesiso
Deny local write aliasestrnval
Don’t deny any local aliasesrefboxtag

But there are modifiers (!, ^, this->), which confuse me a lot. Plus you can have capabilities declared on variables (makes sense), but as well you can have capabilities on functions. I decided to postpone learning refcap because otherwise, I would never finish writing Lisp.

Compiler error messages

If you make an error in one place, the compiler will complain about errors in many places (understandably because it tries to compare a graph and find all points where types don’t fit). As the result, you may have very long error traces. One example:

1interface MallEffectHandler
2  fun read_file(file_name: String): String ?
4class StandardEffectHandler is MallEffectHandler
5  fun read_file(file_name: String) ? =>

I made one error - I didn’t implement the interface correctly (there is no return type). I think it should complain about only one file here, but it as well reported every case where I tried to use StandardEffectHandler in place of MallEffectHandler.

No exceptions

There is error function, which is like Go’s panic, except it doesn’t take any arguments, so you can’t provide an error message.

If you need to provide error messages you will need to develop your own mechanism for it. There are several approaches:

  1. Store the error in the instance of the class method of which caused it. Example: parse_report in JSON parser
  2. “Either monad” (not a monad, just inspired by it). The idea is to have a disjoint union of successful and error cases Either[L,R] = (Error[L]|Result[R]), so you can pattern match and see if it was a success or not
  3. Have one instance of a class that you pass around and which would store the error.

After the trial, I settled with the last approach, because others were doing code much harder than it supposes to be.

This is problematic, because language should provide default features, otherwise the community will invent some ways to work around, which would be incompatible with each other.

No recursive types

There are no recursive types. Instead, I was forced to use classes to workaround. I learned this trick from Json package.

It is kind of works, but still, there are limitations. For example, I have MalList, which represents a Lisp list, which can contain any Lisp values. But then if I would want to have a List of only MalSymbol (to describe a list of function arguments), I can’t force it, I will get back into the problem of recursive types. Instead, I am forced to use the array - Array[MalSymbol].


Lisp is a dynamically typed language, but Pony is statically typed. How do I jump from one world to another? Well, there is a couple of ways. If it would less strict type system, like C, I could just do typecasting, which is not an option in Pony (there is as operator, but it won’t help).

Another way to do it (when you have a more strict type system) is to use something, like Elm ports or io-ts. This is a way to prove to a static type system, that dynamic data is of the correct type. Unfortunately, Pony type system is not expressive enough to make those decoders composable. By composable I mean I would write one decoder, which would prove that value is MalSymbol, then I would write another decoder which would prove that value is an array, then I can combine them to prove that value is an array of symbols.

So I end up writing code like this:

1fun ref as_bool(input: MalType): Bool ? =>
2  match input
3  | let output: Bool => output
4  else
5    _eh.err("Expected bool instead got " + MalTypeUtils.type_of(input))
6    error
7  end

This function will either return Bool or will throw an error, because throwing an error is not considered as type in the return value, for type checker it means it always returns Bool value.

But I need a lot of those functions and I need to use them in a lot of places, which is a bit ugly.


There is no IDE for Pony. I configured VSCode with

  • syntax highlighting
  • remote container - o run Pony compiler in Docker, so I don’t need to install it locally and
  • ctags - for some code navigation. I still need to configure it to rerun on file save with fswatch

But Pony is a statically typed language, so it can have a much nicer IDE experience, like show inferred types on hover, or show documentation on hover, or jump to definition, better autocompletion (aka IntelliSense).

I guess if they bootstrap Pony it would be possible to reuse parser and type checker to build language server.


There is enough documentation, but it is kind of scattered across several sites. Sometimes it is hard to find what you want, but not impossible. I use those links to find all I need:


It was a nice experience and in some way, Pony made me write a bit better-organized code because in a less strict language I could as well hack around some things.

The most interesting part would be to actually use actors (and expose them in a Lisp). Sadly I didn’t get to that part yet, because I was focusing to make my Lisp work and I would need to understand better refcap before I can use actors.

Source code is here.

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