Explicity mention missing constructors of matchWith patterns

[jreut]

When writing pattern-matching functions using matchWith, it is possible to not cover all the variants of a union. Since of course we cannot statically verify all cases of a pattern match are covered, this error occurs at runtime. Currently, attempting to match on a missing variant results in a generic error. I think it would be useful to have a more specific error that tells us what variant we need to add to the matchWith pattern.

I'm more than happy to submit a pull request with this small change if you think it would be useful!

Steps to reproduce

import { union } from 'folktale/adt/union'
const Operation = union({
  Add: (a, b) => ({ a, b }),
  Subtract: (a, b) => ({ a, b }),
})

// evaluate : Operation -> Int
const evaluate = op => op.matchWith({
  Add: ({ a, b }) => a + b
  // Subtract not implemented!
})

evaluate(Operation.Subtract(3, 4))

Expected behaviour

Error like Variant "Subtract" not covered in pattern

Observed behaviour

Error like TypeError: pattern[name] is not a function

Environment

Folktale v2.0.0-rc1

[robotlolita]

Ah, I didn't realise the error was that bad. Yeah, we do need a better error message here.

[robotlolita]

This has been released as folktale@2.3.0