Fix the inconsistent type of "env"

[Avaq]

• gitter/sanctuary#58aebbeae961e53c7f5762af
• Correct type of $.test #121 (comment)

[Avaq]

One of the ideas was to replace all type constructors (Type... -> Type) in env with types that have already been applied with Unknowns, eg: env.concat([Maybe(Unknown), Either(Unknown, Unknown)]).

My concern is that currently, type constructors might actually be used by sanctuary-def in order to create concrete types for values with variable types, otherwise, what's the value of having these type constructors in the environment in the first place?

[davidchambers]

Thanks for opening this issue, Aldwin.

otherwise, what's the value of having these type constructors in the environment in the first place?

So that values such as [1, 2, 3] and Just('foo') are recognized. For example:

const a = $.TypeVariable('a');

$.create({checkTypes: true, env: [$.Number]})('id', {}, [a, a], x => x)([1, 2, 3]);
// ! TypeError: Type-variable constraint violation
//
//   id :: a -> a
//         ^
//         1
//
//   1)  [1, 2, 3] ::
//
//   Since there is no type of which all the above values are members, the type-variable constraint has been violated.

[Avaq]

So that values such as [1, 2, 3] and Just('foo') are recognized

But wouldn't adding Maybe(Unknown) to the env instead of Maybe make the type-checker think that Just(1) and Just('foo') are consistent? Or is some other mechanism used to determine consistency over multiple occurrences of the same type-var?

[davidchambers]

$.Unknown is special. Search the code for UNKNOWN and you'll find several occurrences.

$.Unknown is used internally when the actual type cannot be completely determined. The type of [], for example, is Array Unknown. $.Array($.Unknown) constrains the outer type without constraining the inner type.

$.Unknown is a bit like a stem cell. :)

[Avaq]

Oh, hold on a second! The type constructors are applied to Unknown the moment they are passed into Sanctuary Def!

That clears everything up. Sanctuary Def simply doesn't use them.

[davidchambers]

Yes. What I'm suggesting is that it should be the caller's responsibility to do something like S.EitherType($.Unknown, $.Unknown) rather than for sanctuary-def to take care of it.

[Avaq]

Totally get that now. This change wouldn't even impact the user much, as most of the changes would happen in the internals. It would be a major version for sanctuary-def, but a minor or even patch for sanctuary.

Come to think of it. It would be a patch for sanctuary-def as well, if we wouldn't publish #121!

[Avaq]

We would simply remove the applyParameterizedTypes line from test, which makes it compatible with the signature originally assigned to it before #121. Nothing changes on the surface.

[Avaq]

And then it's just S.env that needs a quick patch.

[davidchambers]

Okay. Let's remove applyParameterizedTypes, release sanctuary-def@1.0.0, then open a Sanctuary pull request to update its sanctuary-def dependency and S.env. Does this sound good?

[Avaq]

Sounds good. I suppose we can close #121 then. Haha, my eagerness to get a patch out actually caused me to have to wait longer.

[Avaq]

Oh wait. If you're going to release this as major, we may as well merge #121 so that people will not get 🐛 bitten in the current version. I don't know.

[davidchambers]

Oh wait. If you're going to release this as major, we may as well merge #121 so that people will not get 🐛 bitten in the current version.

This seems reasonable.

I'm about to put my head down. I'll come back to these issues and pull requests on Saturday. :)

[Avaq]

After collecting my thoughts, this is what I think we should do:

• Patch sanctuary to export an env which is strictly Array Type, immediately fixing the issue at hand.
• Close Correct type of $.test #121
• Open PRs in sanctuary-def to:
  1. Remove the unnecessary call to applyParameterizedTypes
  2. Document how to apply parameterized types in order to create a valid env
• Possibly release the aforementioned changes to sanctuary-def under a new major version. There's no hurry anymore since we patched the problem in sanctuary.