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https://github.com/brendanzab/language-garden

A garden of small programming language implementations 🪴
https://github.com/brendanzab/language-garden

compilation compilers dependent-types elaboration l-systems programming-languages typechecking

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A garden of small programming language implementations 🪴

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# Language garden 🌱



Some toy programming language implementations, mostly implemented in OCaml.



These projects are mostly my attempt to understand different techniques and

approaches to implementing programming languages. Perhaps from these seedlings

something new and interesting might germinate?



## Implementations



### Elaboration



Elaboration is an approach to implementing language front-ends where a complicated,

user friendly _surface language_ is type checked and lowered to a simpler, typed

_core language_. This approach to type checking is particularly popular and

useful for implementing dependently typed programming languages, but is more

widely applicable as well.



Simply typed:



- [**elab-stlc-bidirectional**](./elab-stlc-bidirectional):

  An elaborator for a simply typed lambda calculus that uses bidirectional

  typing to allow some type annotations to be omitted.

- [**elab-stlc-bidirectional-stratify**](./elab-stlc-bidirectional):

  An elaborator that partially stratifies a combined type and term language into

  a simply typed core language.

- [**elab-stlc-abstract**](./elab-stlc-abstract):

  An LCF-style elaborator that moves the construction of well-typed terms behind

  a trusted interface.

- [**elab-stlc-unification**](./elab-stlc-unification):

  An elaborator for a simply typed lambda calculus where type annotations can be omitted.

- [**elab-stlc-letrec-unification**](./elab-stlc-letrec-unification):

  Extends the simply typed lambda calculus with recursive let bindings.

- [**elab-stlc-variant-unification**](./elab-stlc-variant-unification):

  Extends the simply typed lambda calculus with structural variant types,

  inferring types eagerly using constraint based unification.



Polymorphically typed:



- [**elab-system-f-bidirectional**](./elab-system-f-bidirectional):

  An elaborator for a higher-rank polymorphic lambda calculus.



Dependently typed:



- [**elab-dependent**](./elab-dependent/):

  An elaborator for a small dependently typed lambda calculus.

- [**elab-dependent-sugar**](./elab-dependent-sugar/):

  An elaborator for a small dependently typed lambda calculus with syntactic sugar.

- [**elab-record-patching**](./elab-record-patching/):

  An elaborator of a dependently typed lambda calculus with singletons and record patching.



### Compilation



These are related to compilation. Mainly to stack-machines, but I’m interested

in exploring more approaches in the future, and other compilation passes

related to compiling functional programming languages.



- [**compile-arith**](./compile-arith/):

  Compiling arithmetic expressions to stack machine instructions and A-Normal Form.

- [**compile-arithcond**](./compile-arithcond/):

  Compiling arithmetic and conditional expressions to stack machine instructions and A-Normal Form.

- [**compile-closure-conv**](./compile-closure-conv):

  Typed closure conversion and lambda lifting for a simply typed lambda calculus

  with booleans and integers.



### Languages



Miscellaneous programming language experiments.



- [**lang-datalog**](./lang-datalog/):

  A simple Datalog interpreter.

- [**lang-doc-templates**](./lang-doc-templates/):

  A programmable document template language that elaborates to a typed lambda calculus.

- [**lang-fractal-growth**](./lang-fractal-growth/):

  Experiments with using grammars and rewriting systems to model fractal growth.

- [**lang-lc-interpreters**](./lang-lc-interpreters/):

  A comparison of lambda calculus interpreters using different approaches to

  name binding.

- [**lang-shader-graphics**](./lang-shader-graphics/):

  An embedded DSL for describing procedural graphics, based on signed distance

  functions. These can be rendered on the CPU or compiled to GLSL shaders.



### Work in progress projects



While most of the above projects need more work put into them, the following

projects need more work put into them and a more incomplete in comparison.



- [**wip-elab-builtins**](./wip-elab-builtins/):

  An elaborator that supports built-in types and operations.

- [**wip-compile-stlc**](./wip-compile-stlc):

  A type preserving compiler for the simply typed lambda calculus.

- [**wip-compile-stratify**](./wip-compile-stratify/):

  Compiling a dependently typed lambda calculus into a stratified intermediate

  language.

- [**wip-compile-uncurry**](./wip-compile-uncurry/):

  Compiling single-parameter functions to multiparameter functions.



## Background



As I’ve been working in the area of programming languages I’ve often found

myself in the position of:



- Explaining the same idea or technique over and over, but not having a minimal

  example I can point to.

- Re-implementing an existing technique (either from a paper, or based on some

  other existing code I’ve seen) in my own way, as a way of learning and

  understanding it more deeply.

- Wanting a place to experiment with an approach before committing to using it

  in a larger project, which can take time and may amount to nothing.

- Trying to recall a technique I’d spent time learning a long ago.

- Having an idea for a small language experiment that does not need to be part

  of a standalone project, but may require some build system setup.



My hope is that by collecting some of these projects and experiments together

into a single repository they might be useful to others and my future self.



The metaphor of a “garden” as related to knowledge work was inspired by the rising

popularity of “digital gardening” (which apparently originates from

[Hypertext Gardens](https://www.eastgate.com/garden/Enter.html)).

While this project is less directly interconnected than other digital gardens,

I still like the idea of each project being a “seedling” that can be nurtured

and tended to over an extended period of time,

with the learning from one project being transferred to the others.

Perhaps a “language nursery” would have been a more fitting name.



I’ve also been particularly inspired by Mark Barbone’s [small, self-contained gists](https://gist.github.com/mb64/)

implementing small type systems and solvers, and Andras Kovacs’ excellent

[elaboration-zoo](https://github.com/AndrasKovacs/elaboration-zoo/) (which was

instrumental in helping me get my head around how to implement elaborators).



If you like this repository, you might find these interesting as well:



- [andrejbauer/plzoo](https://github.com/andrejbauer/plzoo/):

  Andrej Bauer’s minimnal programming language demonstrations

- [AndrasKovacs/elaboration-zoo](https://github.com/AndrasKovacs/elaboration-zoo/):

  Minimal implementations for dependent type checking and elaboration by Andras Kovacs

- [mspertus/TAPL](https://github.com/mspertus/TAPL): Updated type system

  implementations from Benjamin Pierce's “Types and Programming Languages”

- [pigworker/Samizdat](https://github.com/pigworker/Samizdat):

  Conor McBride’s programming scrapbook

- [tomprimozic/type-systems](https://github.com/tomprimozic/type-systems)

  Implementations of various type systems in OCaml

- [RiscInside/LanguageEtudes](https://github.com/RiscInside/LanguageEtudes/):

  Single-file typechecker/interpreter/compiler implementations



## Conventions and style choices



### Provide lots of type annotations



The predominant style in OCaml of leaving off type annotations

makes understanding and porting code far more difficult.

Instead I try to add type annotations to most top-level type signatures.



### Avoid opening modules



When `open` is used I find it hard to figure out where identifiers are coming from without an editor.

Instead I prefer using an explicitly qualified path where possible.



### Group related variants with a common prefix



In the past I’ve often found it hard to find related nodes in an AST

when trying to understand other people’s code.

For example, the following variants might all refer to different parts of a dependent pair type:



```ocaml

type tm =

  ...

  | Sig of string * tm * tm

  | Pair of tm * tm

  | Fst of tm

  | Snd of tm

```



Instead I prefer to use the following constructors:



```ocaml

type tm =

  ...

  | PairType of string * tm * tm

  | PairLit of tm * tm

  | PairFst of tm

  | PairSnd of tm

```



### Use types to disambiguate variant names



OCaml’s variant constructors aren’t namespaced under the type like in Rust or Lean,

so reusing the same variant name will result in ambiguities

if you are relying on global type inference.

Generally OCaml programmers will either:



1. Wrap every type in a module

2. Come up with an ad-hoc prefix for to prevent the conflict



I find the former convention often results in duplicated datatype definitions

(mutually dependent modules require explicit module signatures),

and the latter is a little arbitrary and ugly.



Instead I’ve decided to just disambiguate variants using the type.

I realise this might make the code a more difficult to understand

and if I come up with a better compromise I might revisit this in the future.



## Development setup



### With Nix



Using [Nix] is not required, but can be useful for setting up a development

shell with the packages and tools used in this project. With [Nix flakes]

enabled:



```sh

nix run .#arith -- compile --target=anf <<< "1 + 2 * 27"

```



[nix-direnv] can be used to load development tools into your shell

automatically. Once it’s installed, run the following commands to enable it in

the project directory:



```sh

echo "use flake" > .envrc

direnv allow

```



You’ll want to locally exclude the `.envrc`, or add it to your global gitignore.



After that, [dune] can be used to build, test, and run the projects:



```sh

dune build

dune test

dune exec arith -- compile --target=anf <<< "1 + 2 * 27"

```



[dune]: https://dune.build

[Nix]: https://nixos.org

[Nix flakes]: https://nixos.wiki/wiki/Flakes

[nix-direnv]: https://github.com/nix-community/nix-direnv



### With opam



Alternatively, [opam] package definitions are provided in the [`./opam`](./opam)

directory. They drive the Nix flake, so _should_ be up to date. I don’t use opam

however, so I’m not sure what the workflow is.



[opam]: opam.ocaml.org