bring your own environment

[davidchambers]

Problem:

S.I(['foo', 42]);
// ! TypeError: Type-variable constraint violation
//
//   I :: a -> a
//        ^
//        1
//
//   1)  ["foo", 42] :: Array ???
//
//   Since there is no type of which all the above values are members, the type-variable constraint has been violated.

If this is a case of array-as-list, a type error is appropriate. But perhaps it's a case of array-as-tuple?

sanctuary-def could—and should—provide Pair a b to support two-element arrays. The fact remains, though, that Sanctuary's environment cannot possibly include every type, as there are infinitely many.

Solution:

The user must have the ability to provide her own environment.

"Initializing" Sanctuary is currently done in one of two ways:

const S = require('sanctuary');

const S = require('sanctuary').unchecked;

If the user is to provide an environment, it makes sense to take (checkTypes :: Boolean, env :: [Type]) as $.create does:

const sanctuary = require('sanctuary');

const S = sanctuary.create(true, sanctuary.env);

const sanctuary = require('sanctuary');

const S = sanctuary.create(false, sanctuary.env);

When @Avaq and I discussed this on Gitter, he suggested having const S = require('sanctuary'); continue to work as it does today. This strikes me as a great idea, as it would allow new users to have a positive first experience with the library.