Inferring parameter types with `auto`

[jarble]

In C++, it is possible to infer parameter types and return types of functions using the auto keyword:

auto square(auto x){
   return x*x;
}

I tried to use this keyword in Carbon, but this program didn't compile successfully:

package sample api;

fn Square(x: auto) -> auto {
  return x * x;
}

fn Main() -> i32 {
  return Square(12);
}

Will this feature be supported in Carbon?

[matthew-russo]

The plans for type inference are defined here: https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/type_inference.md https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design#auto https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design#auto-return-type

~~which says its currently limited in scope to variable declarations and function return types so function parameters seem to be out of scope.~~

The current design covers variable declarations and function return types. Type inference for function parameters is not fully designed or decided upon yet.

the following variations work with the current explorer

package sample api;

fn Square(x: i32) -> auto {
  return x * x;
}

fn Main() -> i32 {
  return Square(12);
}

package sample api;

fn Square(x: i32) -> i32 {
  var i: auto = x * x;
  return i;
}

fn Main() -> i32 {
  return Square(12);
}

package sample api;

fn Square(x: i32) -> auto {
  var i: auto = x * x;
  return i;
}

fn Main() -> i32 {
  return Square(12);
}

[chandlerc]

function parameters seem to be out of scope

Not quite.

For variables and return types there is typically a specific expression to infer the type from, and so we call this type inference.

For parameters, the more useful thing is to make the function generic (or even templated) over the type so that it can be used with different types.

So there has been some informal discussion of having:

fn F(x: auto) { ... }

Be essentially sugar for:

fn F(x: (template _:! Type)) { ... }

Or if we disallow nested bindings in patterns (something I think we're leaning towards), then for some made up name __T:

fn F[template __T:! Type](x: __T) { ... }

I'm not sure this has been fully fleshed out and added to the design at this stage though, much less fully implemented in the explorer.

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