Feature request: Allow type to return void

[mratsim]

Currently you can alias the void type but the type proc cannot return void, it's a compile-time error.
There might be a good reason for it in that case it should be mentionned in the docs, otherwise I think it is a useful feature, here is a related post by @cooldome on the forum.

import macros,typetraits


type v = void # This works
echo v.name

type w = type((echo "foo"))  # This fails

# I want to followup with "when w is void:"

# Use case, so that we can `when test_type is void`
# in the following template that is used often to iterate
# and transform sequences (or any collection)
template test(b: seq[int], loopBody: untyped): typedesc =

  template test_type(): untyped =
    type((
      block:
        var x{.inject.}: type(items(b));
        loopBody
      ))

  when compiles(test_type()):
    # `when test_type() is void:` would be better I think
    test_type()
  else:
    void

let a = @[0, 1, 0, 1]

echo test(a, x.bool).name      # bool
echo test(a, echo x.bool).name # void

[mratsim]

Also this would allow to distinguish between those two cases:

Void type

let a = @[1, 2, 3]
let b = @[true, true, false]

type((
  block:
    var x: type(items(a))
    var y: type(items(b))
))

Statement with no type

let a = @[1, 2, 3]
let b = @[true, true, false]

type((
  block:
    var x: type(items(a))
    var y: type(items(b))
    if y:
      x
))

[krux02]

I agree, that this is a good idea.

[Araq]

When I looked at void's problems I wanted to erase it from Nim altogether, it's that bad.

[krux02]

You want to force every function to return a value?

[andreaferretti]

It could be a type like any other. Scala, for instance, has a type Unit, whose only member is the object denoted by the literal (). As such it carries no information, and it could be deleted behind the scenes, but treated like any other type elsewhere

[mratsim]

It can be called, void, Empty or NoneType but fundamentally I don't see how removing it works for a typical program or testing the absence of return values.

[Araq]

See nim-lang/RFCs#508 for details.

[metagn]

This is now implemented (since 1.6), #17807