Inko Progress Report: March 2022

Type checking progress

The new type checker is complete, and is able to check Inko's standard library without producing any errors or overlooking incorrect code. Testing is still done manually at this stage, and this will probably remain so until our mid-level IR is complete.

Inko's mid-level intermediate representation

With the type checker fully implemented, we started working on Inko's mid-level IR, or "MIR" for short. This IR is used for more advanced analysis (e.g. ensuring a variable isn't used after it's moved elsewhere) and optimisations. The IR is a graph based IR, with an instruction set that resembles the virtual machine's instruction set.

Various operations that are implicit in the AST/syntax are explicit in MIR. For example, you can pass an owned value to an argument that expects a reference, and Inko will automatically pass the value as a reference then drop the owned value after the call. The result is that code such as this:

fn foo(value: ref String) {
  ...
}

fn bar {
  foo("Alice")
}

Is the same as this:

fn foo(value: ref String) {
  ...
}

fn bar {
  {
    let _val = "Alice"

    foo(ref _val)
  }
}

In MIR, this translates to more or less the following:

fn foo(value: ref String) {
  ...
}

fn bar {
  let _v1 = "Alice"

  foo(ref _v1)
  drop(_v1)
}

When lowering the high-level IR/AST to MIR, we also apply other changes such as:

• Desugaring closures into classes
• Desugaring enum classes into regular classes
• Desugaring pattern matching into more primitive expressions (e.g. simple equality checks)
• Turning named arguments into positional arguments
• Constant folding constant definitions, meaning let A = 10 + 5 is turned into let A = 15

Future optimisations we'll apply on MIR will include inlining, devirtualisation, constant folding and propagation (inside method bodies), dead code elimination, and more.

Plans for April

In April we'll continue work on MIR, and we expect some of this work to carry over into May. If you'd like to follow along with the progress made, we recommend joining the Matrix channel or the #inko channel in the /r/ProgrammingLanguages Discord server. If you'd like to support Inko financially, you can do so using GitHub Sponsors.