const Foo=int compiles; is that legal? what does it mean?

[timotheecour]

EDIT: is that the syntax to get a const typedesc variable ? where is that documented?

const Foo=int
echo Foo is int # true
echo int is Foo # Error: type expected

[metagn]

It's equivalent to a typedesc value in the VM.

import macros

const Foo = int

macro foo(name: untyped): untyped =
  newTree(nnkVarSection, newIdentDefs(name, getType(Foo), newLit(5)))

foo(bar)
echo bar * 3 # 15

I don't think it's documented though it sometimes would be expected to work.

[Araq]

It's not legal.

[ghost]

Related?

proc test(a = int, string = string) =
  echo 1

test()

[GULPF]

@Tim-St No, that's a default value of a typedesc argument.

import typetraits
# Same as 'proc test(a: typedesc = int): string = $a'
proc test(a = int): string = $a
doAssert test() == "int"
doAssert test(string) == "string"

[ghost]

Ok, so If I understood correctly, const Foo = int should not be legal but
proc test(Foo = int) = discard should be legal?
Now one could argue, Foo in proc is not const, but then why does var Foo = int not work?

[dom96]

Now one could argue, Foo in proc is not const, but then why does var Foo = int not work?

Because procedures can be generic, variables can't.

[ghost]

But const Foo = int should be illegal and var Foo = int is already illegal and also a proc param (afaik) is always var or const. So I don't really understand why it should be allowed in the context of proc arguments when it's disallowed everywhere else, do you have an example where it makes sense to allow it in a proc but disallow it everywhere else?

Edit:
Well maybe for something like this

proc test(a = typedesc) =
  when a is int:
    echo 123
  else:
    echo "abc"

But still I think it is better to have it consistent.

Edit 2:
Ok, the style like in the proc is used in the nim compiler, so I assume it's legal syntax.

[Clyybber]

@Tim-St types != values. Typedescs bridge that gap, and allow types to be used as compile-time values. That only makes sense for proc arguments.

[mratsim]

As I said in #8657. const Foo=int should be illegal at all time. type Foo=int should cover the same use-cases without special casing const for typedesc.