4 ms·

Helpful explanation. I think a lot of the trouble for me in understanding GADTs is the Haskell syntax used in a lot of examples. My static typing experience is

Helpful explanation. I think a lot of the trouble for me in understanding GADTs is the Haskell syntax used in a lot of examples. My static typing experience is limited to TypeScript and Rust, so I'm curious what a GADT would look like if they were added to those languages.

lalaithion 5y ago
Here's all the code from the parent in Rusty pseudosyntax: enum MyRecord { ConstructorName(Int, String, Bool) } enum TrafficLight { Red, Yellow, Green } enum AST { Integer(Int), Boolean(Bool), Addition(Ast, Ast), Multiplication(Ast, Ast), Equals(Ast, Ast), Not(Ast) } `Not(Equals(Integer(1), Integer(2))) : AST` enum IntErr { Error(String), IntResult(Int) } enum StringErr { Error(String), StringResult(String) } enum WithError<A> { Error(String), Result(A) } `Result(3) : WithError<Int>` `Result(True) : WithError<Bool>` enum Weird<A, B> { IgnoreTheType(Int), } `Weird<String, Bool>` `IgnoreTheType(1) : Weird<String, Bool>` `IgnoreTheType(2) : Weird<Char, Float>` `IgnoreTheType(3) : Weird<Bool, Double>` `Addition(Boolean(True), Not(Integer(3)))` enum AST<R> { Integer(Int), Boolean(Bool), Addition(Ast<Int>, Ast<Int>), Multiplication(Ast<Int>, Ast<Int>), Equals(Ast<R>, Ast<R>), Not(Ast<Bool>) } Finally, we get our GADT: enum AST<R> { Integer(Int) : Ast<Int>, Boolean(Bool) : Ast<Bool>, Addition(Ast<Int>, Ast<Int>) : Ast<Int>, Multiplication(Ast<Int>, Ast<Int>) : Ast<Int>, Equals(Ast<R>, Ast<R>) : Ast<Bool>, Not(Ast<Bool>) : Ast<Bool> }
- bradrn 5y ago
  Thanks lalaithion! I don’t know Rust, so it’s interesting to see how different it is compared to Haskell. A question: how much of this would actually compile in Rust?
  - deleted 5y ago
    [deleted]
  - kaoD 5y ago
    - Assuming `Int` and `Bool` are user-defined types (Rust has `bool` and many types of ints like `usize`, `i64`, etc. instead). If you want to work over ANY primitive integer you can use the `num_traits` crate and make your type generic over `num_traits::PrimInt`, or even `num_integer::Integer` to include support for crates such as `num_bigint`. - Rust needs indirection for recursive data types (like the AST) using `Box`, references, or another kind of indirection. Otherwise, the size isn't known at compile time (since it's potentially infinite). - Rust doesn't have GADTs (yet?) so the last AST is purely theoretical. - `IgnoreTheType` (and `AST<R>`) would require explicit use of `PhantomData` for variants without `R` https://doc.rust-lang.org/std/marker/struct.PhantomData.html https://doc.rust-lang.org/std/marker/struct.PhantomData.html On Rust's playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=40b9ce0351c3adccd53cc6231c72d742 https://play.rust-lang.org/?version=stable&mode=debug&editio...
    - bradrn 5y ago
      Thank you! So it looks like most of my post does indeed work, except for the GADT itself. (Also, I had not known about Rust Playground, so thanks for introducing me to that too.)
- deleted 5y ago
  [deleted]

lalaithion 5y ago

Here's all the code from the parent in Rusty pseudosyntax: enum MyRecord { ConstructorName(Int, String, Bool) } enum TrafficLight { Red, Yellow, Green } enum AST { Integer(Int), Boolean(Bool), Addition(Ast, Ast), Multiplication(Ast, Ast), Equals(Ast, Ast), Not(Ast) } `Not(Equals(Integer(1), Integer(2))) : AST` enum IntErr { Error(String), IntResult(Int) } enum StringErr { Error(String), StringResult(String) } enum WithError<A> { Error(String), Result(A) } `Result(3) : WithError<Int>` `Result(True) : WithError<Bool>` enum Weird<A, B> { IgnoreTheType(Int), } `Weird<String, Bool>` `IgnoreTheType(1) : Weird<String, Bool>` `IgnoreTheType(2) : Weird<Char, Float>` `IgnoreTheType(3) : Weird<Bool, Double>` `Addition(Boolean(True), Not(Integer(3)))` enum AST<R> { Integer(Int), Boolean(Bool), Addition(Ast<Int>, Ast<Int>), Multiplication(Ast<Int>, Ast<Int>), Equals(Ast<R>, Ast<R>), Not(Ast<Bool>) } Finally, we get our GADT: enum AST<R> { Integer(Int) : Ast<Int>, Boolean(Bool) : Ast<Bool>, Addition(Ast<Int>, Ast<Int>) : Ast<Int>, Multiplication(Ast<Int>, Ast<Int>) : Ast<Int>, Equals(Ast<R>, Ast<R>) : Ast<Bool>, Not(Ast<Bool>) : Ast<Bool> }

bradrn 5y ago
Thanks lalaithion! I don’t know Rust, so it’s interesting to see how different it is compared to Haskell. A question: how much of this would actually compile in Rust?
- deleted 5y ago
  [deleted]
- kaoD 5y ago
  - Assuming `Int` and `Bool` are user-defined types (Rust has `bool` and many types of ints like `usize`, `i64`, etc. instead). If you want to work over ANY primitive integer you can use the `num_traits` crate and make your type generic over `num_traits::PrimInt`, or even `num_integer::Integer` to include support for crates such as `num_bigint`. - Rust needs indirection for recursive data types (like the AST) using `Box`, references, or another kind of indirection. Otherwise, the size isn't known at compile time (since it's potentially infinite). - Rust doesn't have GADTs (yet?) so the last AST is purely theoretical. - `IgnoreTheType` (and `AST<R>`) would require explicit use of `PhantomData` for variants without `R` https://doc.rust-lang.org/std/marker/struct.PhantomData.html https://doc.rust-lang.org/std/marker/struct.PhantomData.html On Rust's playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=40b9ce0351c3adccd53cc6231c72d742 https://play.rust-lang.org/?version=stable&mode=debug&editio...
  - bradrn 5y ago
    Thank you! So it looks like most of my post does indeed work, except for the GADT itself. (Also, I had not known about Rust Playground, so thanks for introducing me to that too.)
deleted 5y ago
[deleted]