Declaration merging problem reproduction

This repository demonstrates a problem when linking two packages that use a library that makes heavy use of declaration merging.

Environment

Given two packages /app and /lib in a monorepo, Yarn workspaces lets us link them:

{
  "name": "app",
  "depencencies": {
    "lib": "workspace:*"
  }
}

We use Yarn because one package (/app) requires the node_modules folder to reside inside the package's folder (a restriction of Meteor, the framework we use). NPM does not provide the option to do this at the moment.

Preparation

Make sure Yarn is installed. Then install all dependencies with:

yarn

Then build the /lib package:

npx tsc -p lib/tsconfig.json

Demonstration

When we now try to build the /app package:

npx tsc -p app/tsconfig.json

TypeScript will fail with the following error:

$ npx tsc -p app/tsconfig.json
lib/box.ts:18:40 - error TS2344: Type '"Box"' does not satisfy the constraint 'keyof URItoKind<any>'.

18 export const Pointed: pointed.Pointed1<URI> = {
                                          ~~~

Found 1 error in lib/box.ts:18

Discussion

The problem is dependent on the import order of the library fp-ts. In /app/index.ts:

1. If /lib is imported before fp-ts, the build works
2. If fp-ts is imported before /lib, the build fails

The reason for the failure is that fp-ts makes heavy use of declaration merging for providing higher-kinded types (HKT). This fails if one kind of HKT is imported in /lib and another in /app, because two different instances of fp-ts will be used.