https://github.com/dannywillems/ocaml-gadt-playground

Play with GADT in OCaml
https://github.com/dannywillems/ocaml-gadt-playground

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Play with GADT in OCaml

• Host: GitHub
• URL: https://github.com/dannywillems/ocaml-gadt-playground
• Owner: dannywillems
• Created: 2023-09-01T12:23:38.000Z (about 1 year ago)
• Default Branch: main
• Last Pushed: 2023-09-01T13:31:28.000Z (about 1 year ago)
• Last Synced: 2024-05-02T01:51:19.182Z (7 months ago)
• Language: OCaml
• Size: 4.88 KB
• Stars: 2
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Play with GADT in OCaml

## Motivation

Over the years, I've seen a few patterns of GADT usecases. For instance,

a stack based and typed language like

[Michelson](https://tezos.gitlab.io/active/michelson.html). In particular, the

GADT offers the safety, at compile-time, that an instruction with two operands

won't be applied if the stack is empty or contains only one element.

It is a nice usecase of a GADT as it leverages runtime checks, and delegates the

work to the compiler. It enforces the user to have called specific instructions

to change the state (= the runtime value) of the type before applying a new

instructions. In a sense, it is useful to encode a typed computation.

Another pattern I encounter is the "finalization" pattern. A type `t` contains

some values, and only when the type `t` reaches a certain state (i.e. a

`finalized` state), the user can perform certain operations. For instance,

loading test parameters before running a test.

We also want to be able to call some functions on the type `t` independently of

its state, for instance to show the current state to log it.

## This work

This work contains a relatively simple interface to show how to avoid a runtime

check using a type-level construction when some data can be in different states.

An application can be a type whose runtime values must be in a certain state

before being used in some other functions.

See:

- [`gadt_playground.mli`](./lib/gadt_playground.mli) for a simple interface.

- [`gadt_playground.ml`](./lib/gadt_playground.ml) for an implementation.

- [`main.ml`](./bin/main.ml) for an example.

## Examples

```ocaml

(* This is a valid program *)

let () =

  let s = create () in

  let s = fill s [4] in

  let s = fill s [5] in

  let s = finalize s in

  show s

```

```ocaml

(* This is not a valid program,

   the finalize method has to be called before calling

   show *)

let () =

  let s = create () in

  let s = fill s [4] in

  let s = fill s [5] in

  show s

```

## Install & Test

```

opam switch create ./ 5.0.0

opam install dune

dune build

dune exec ./bin/main.exe

```