remove implicit support for values of "foreign" types

[davidchambers]

#38 provided a partial solution for sanctuary-js/sanctuary#165. sanctuary-js/sanctuary#206 offers a more complete solution, so this special case now seems unnecessary:

    //  If none of the values is a member of a type in the environment,
    //  and all the values have the same type identifier, the values are
    //  members of a "foreign" type.
    if (isEmpty(filterTypesByValues(env, values)) &&
        all(values.slice(1), $$typeEq($$type(values[0])))) {
      //  Create a nullary type for the foreign type.
      return [type0($$type(values[0]))];
    }

[Avaq]

This would mean people have to get into sanctuary-def when they want to use a foreign value, right? What happens when people just try to input a foreign type? Maybe it's best to leave the check in and raise a descriptive error when it happens.

[davidchambers]

Excellent point, Aldwin. It would be unfortunate to force a user to learn about custom environment and type definitions in order to apply S.I to document.body. So while the code quoted above is a special case, it is perhaps a justifiable special case.

I'm pleased I extracted this change from the changes I made over the weekend in an effort to improve performance. Had I not done so, this important change may have slipped through without discussion.

I'm now undecided. I'll leave this pull request open for a while in case I'm able to convince myself that removing the special case is a good idea.

[davidchambers]

I'll leave this pull request open for a while in case I'm able to convince myself that removing the special case is a good idea.

I have convinced myself that this is a good idea. Subsequent efforts to improve performance and add functionality have been hindered by the need to preserve this special case. I'm therefore motivated to remove this special case so I can continue to make progress.

It would be unfortunate to force a user to learn about custom environment and type definitions in order to apply S.I to document.body.

I now believe this to be the correct approach:

const {create, env} = require('sanctuary');
const domTypes = require('sanctuary-dom-types');

const S = create({checkTypes: true, env: env.concat(domTypes)});

[kedashoe]

const {create, env} = require('sanctuary'); const domTypes = require('sanctuary-dom-types'); const S = create({checkTypes: true, env: env.concat(domTypes)});

looks good to me