[ghc-steering-committee] Proposal: Embrace Type in Type

[Richard Eisenberg]

> On Feb 1, 2018, at 8:58 PM, Joachim Breitner <mail at joachim-breitner.de> wrote:
> 
> In particular in light of our use of “type” as a explicit namespace
> token – so far in export and import lists and fixity declarations – I
> worry that we will prevent ourselves from using more such explicit
> namespace things in the future.

This is a really good point. In private musings, I've often wondered about using the keyword `type` in expressions to denote a namespace change. More concretely, I'm worried about the Dreaded Namespace Problem (DNP): that is, a dependently typed language does not want to have separate type-level and term-level namespaces. Of course, Haskell has this.

The best solution I have so far to the DNP is to introduce a *new* namespace, distinct from the two namespaces we have so far. Let's call it the "default" namespace. When -XDependentTypes is on, the default namespace is in effect. Name lookup looks first in the default namespace. If that fails the namespace consulted next depends on context: the "data" namespace in terms and the "type" namespace in type signatures. (This last bit needs to be specified more concretely, but you get the idea.) Or, perhaps, a failed lookup in the default namespace will look up in both the type and data namespaces, erroring if a name appears in both.

If a user wants to specify a namespace directly, they have a very easy way to do so: `type Foo` will look in the type namespace, `data Foo` will look in the data namespace, and `default Foo` will look in the default namespace. :) Because these keywords make sense currently only at the beginning of a line, this Just Works. I also imagined these constructs could scope over a subexpression: `type (T S)`.

All of this deserves a proper proposal and it's too early for that proposal. Nevertheless, I'm grateful that Joachim said something here, given that adding `type` as a spelling of `Type` would invalidate this approach. I also am swayed by the fact that `Type` isn't fully primitive, and making a keyword mean something that's not primitive is a bit awkward. I thus retract this proposed alternative and will update the proposal accordingly.

Richard