First of all thanks for all the feedback and looking into it, appreciate it!

Yeah GRIN is a great project, it took a lot of debugging and analysis to make it compile 100% especially with monomorphization involved.

I'll look into Unicode support, makes total sense. Didn't scope it in initially. I can fix the site ligatures too, that's a fair remark.

> I don't really understand why you have an IO monad. The language isn't pure - `.exec()` means any function can perform IO actions no matter its type signature - so what's IO really for?

That's a fair point, I still left a place for `.exec()` to happen as un-handled side-effect. But the preference is with using the IO monad as the stdlib is built around it, with `main() -> IO[i32]` as a type signature. As languages evolves I'm planning to build a runtime around IO execution, and build more constraints for handling strict side-effects. However for this initial stage of the language, I left it as a really simple solution.

> Do `impl` additions export? What happens when two libraries add the same function name with different signatures (or just bodies!) to a type's `impl` ?

For now the language doesn't support modules (libraries), I'm planning on adding it. At the moment it's a bit of undefined behavior, as overloading would occur with latest `impl` definition.

> Is currying automatic? It doesn't seem to be, but, eg, the `sum(x: i32, y: i32)` function theoretically could be called as `sum(5)` to create a closure, but this isn't a documented feature if so.

In the type-system it is automatic, and it successfully passes type checker as it's entirely built on top of lambda calculus. But there's an issue with codegen right now. I can def look into it and document it.

`Self` isn't the applied type (`List[A]`), rather it's the type constructor of kind `* -> *` constrained by `Functor`. In the map example it gets desugared into:

  fun map[Self: Functor, A, B](self: Self[A], f: A -> B) -> Self[B];

Since `Self` is the unapplied constructor, `Self[B]` just means `Functor[B]` e.g. `List[B]` not `List[A][B]`.

The example you've shown with `SizedFunctor` is not currently supported, as support for associated types is not yet implemented. I got it on the roadmap tho!

    trait ConvertTo[T]:
      fun convert(self) -> T;

    trait Functor:
        fun fmap[A, B](f: A -> B, c: Self[A]) -> Self[B];

     ...

    impl Functor for List[A]:
        fun fmap[A, B](f: A -> B, l: List[A]) -> List[B]
            List::fold(l, Nil[B], (t, h) => Cons(f(h), t))

    ...

This is great, I haven't been introduced in the notion of supercompiliation. Reading the papers you've listed and going through GRIN's paper [1] it ticks the boxes in terms of laziness and graph reduction. To keep the implementation of Fuse simple I've decided on using strict evaluation of GRIN programs instead of laziness, with my assumption that it would harder to debug/reason on the program. However, this was one of my next improvements: switching to a lazy evaluation with similar semantics to Haskell programs.

> It appears that Fuse does not have user-defined operators. Am I right?

Not yet, but I left this mechanism completely open. As operators are defined as type classes with their signs as method definitions.

  [1] http://nbviewer.jupyter.org/github/grin-compiler/grin/blob/master/papers/The%20GRIN%20Project.pdf

Can you expand on your understanding of GRIN doing parts of supercompilation? As I understand it, GRIN doesn't do any supercompilation; it's a structural transformation optimiser built for functional languages, analyzing program flow across function calls for the whole program at once.

As I understand supercompilation, it's an extension of partial evaluation - optimisation is done on a graph of possible execution traces. The downsides should be obvious: execution traces rapidly grow massive, compilation resources grow superlinearly, and there are many cases in which the result is worse than the original.

What value would Fuse get from equivalence of terms, do you think?

OCaml has both compilator and interpreter.

Lua isn't statically typed, and while there is Teal – a statically typed variant of Lua – it requires separate build step.

Haskell: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/runghc.ht...

F# is good because it can easily fetch and import third party dependencies in a script.

I actually can't think of any functional languages that use < > for type variables (I'm sure there's one or two, but it can't be common)

This is only true for C-style languages? Flix uses square brackets, as does Effekt, and Nim, as well as Python. Probably many more I'm missing. Gleam even uses normal parens.

I'm not sure most languages get a vote here.

They can't even represent Fuse's Functor example, i.e.

  f: A -> B, x: F[A]

so maybe they don't get any points for syntax.

Scala and a few others use `[]` (the compiler itself is written in Scala)

As I was deciding on the compiler backend, I stumbled upon on it in r/ProgrammingLanguages on reddit. I liked the syntax itself and the fact I can compile the language in the IR of a mini functional language; with a lot of benefits in terms of optimizations. Especially as (strict) pure functional language are notoriously slower than imperative language because of lack of mutations. Allowing me to have a higher-order functional language that has zero cost abstractions.

Technically yes, although there's no support for FFI yet. Purity would allow it as long as side-effects are wrapped into IO type.

Yeah the LSP support is next, my goal is to implement the language server in the fuse itself. At the moment there’s a simple formatter implementation fusefmt: https://github.com/fuselang/fuse/blob/master/examples/fusefm..., you can compile it with fuse and hook-it with your editor.

For example I’ve this config in helix:

  [[language]]                                                                                                                       
  name = "fuse"                                                                                                                      
  scope = "source.fuse"                                                                                                              
  file-types = ["fuse"]                                                                                                              
  injection-regex = "fuse"                                                                                                           
  comment-token = "#"                                                                                                                
  indent = { tab-width = 2, unit = "  " }                                                                                            
  auto-format = true                                                                                                                 
  formatter = { command = "fusefmt" }                                                                                                
                                                                                                                                     
  [[grammar]]                                                                                                                        
  name = "fuse"                                                                                                                      
  source = { git = "https://github.com/stevanmilic/tree-sitter-fuse", rev = "eb5698f4867a4192064e54a92be280f4d2130e03" }

Haskell is a great language with a really advanced type-system, although I found its syntax hard to read at times especially as I was exploring the language at first. On the other hand I really liked how Rust syntax was defined in terms of ADTs, Traits & Methods Impls, with type signatures required for functions. Hence I wished for a similar functional language that has such write-style and type concepts, but stripping away the borrow checker, mutations, etc.