typescript parser proof of concept

goal

This proof of concept demonstrates the ability to re-use the Typescript typechecker on an AST that was not generated by the typescript parser (but perhaps another parser altogether).

setup

there are two programs fake-parser and typescript-parser that analyse the same program and use the typechecker to infer types of variables. the fake-parser will do so based on an AST, the typescript-parser will do so based on a source file.

Both programs analyse the same source code and print out the same variables.

input

click to reveal source code

const cloud: {
  CreateBucket: <T>() => Bucket<T>;
} = {} as any;

interface Bucket<T> {
  on_object_created(handler: (key: string, obj: T) => void);
}

enum Kind {
  Cat,
  Dog
}

interface Cat {
  kind: Kind.Cat;
  catName: string;
  age: number;
}

interface Dog {
  kind: Kind.Dog;
  dogName: string;
  age: number;
}

const bucket = cloud.CreateBucket<Cat | Dog>();
bucket.on_object_created((_key, obj) => {
  const catName_0 = obj.catName;
  const dogName_0 = obj.dogName;
  const age_0 = obj.age;

  if (obj.kind === Kind.Cat) {
    const catName_1 = obj.catName;
    const dogName_1 = obj.dogName;
    const age_1 = obj.age;
  } else {
    const catName_2 = obj.catName;
    const dogName_2 = obj.dogName;
    const age_2 = obj.age;
  }
});

output

The output of either program is expected to be:

before narrowing to either Cat or Dog
variable catName_0  has type error
variable dogName_0  has type error
variable age_0      has type number

after narrowing to Cat
variable catName_1  has type string
variable dogName_1  has type error
variable age_1      has type number

after narrowing to Dog
variable catName_2  has type error
variable dogName_2  has type string
variable age_2      has type number

executing the test

from the root of this project, run the following two commands and compare the results

• ts-node src/fake-parser/test.ts
• ts-node src/typescript-parser/test.ts