Namespacing for data constructors

[d-xo]

The current implementation places all data constructor names in the global scope. We do not check for duplicated data constructor names, and in the case where duplicates are present, one overwrites the other:

Fails

data Foo = Baz;
data Bar = Baz;

function main() {
  let x = Baz;
  let y : Foo = Baz;
}

Types do not match: Bar and Foo
 - in:let y : Foo = Baz ;
 - in:function main () {
   let x = Baz ;
   let y : Foo = Baz ;
}

Accepted

data Foo = Baz;
data Bar = Baz;

function main() {
  let x = Baz;
  let y : Bar = Baz;
}

Solution

Implement a namespace for all data constuctors (e.g. Foo.Baz and Bar.Baz) allowing duplication. Consider sugar (e.g. The . operator in lean, or c++ style ADL) to allow omitting the namespace qualifier in cases where a unique namespace can be inferred from the context.

[mbenke]

OTOH with repeated type constructors ...

data Foo = Bar;
data Foo = Baz;

function main() {
  let x = Baz;
}

we get

Undefined constructor:
Baz
in type:
Foo

 - in:function main () {
   let x = Baz ;
}

while Haskell says

foo.hs:2:1: error: [GHC-29916]
    Multiple declarations of ‘Foo’
    Declared at: foo.hs:1:1
                 foo.hs:2:1