playing around with typescript

stress_test.ts

// a First is a function which takes two arguments
// of any type, and returns the type of the first argument
type First<T, U> = (a: T, b: U) => T

// as a non-trivial example, add_timedelta(time, timedelta) returns time

// doubler is parameterized on <T, U>, passes those parameters through to First
// typescript can figure out that f(f(a, b), b) is type correct
// since f is of type First, then it will return the same type as its first argument
// note we don't have ANY concrete types yet
let doubler: <T, U>(f: First<T, U>, a: T, b: U) => T = (f, a, b) => f(f(a, b), b)

// f takes a string and a number, and returns a string e.g. ("a", 1) => "a1"
let f:  (a: string, b: number) => string = (a, b) => a + b

// doubler knows NOTHING about F until this point
// at this callsite, doubler becomes a concrete type, T and U above becomes
// string and number
const result = doubler(f, "a", 1)

console.log(result)

// let's make the compiler work harder
// we will provide NO nominative types
// it needs to figure out that the lambda (a + b)
// will be passed "a" and 1, therefore it conforms
// to the shape of First we defined above
doubler((a, b) => a + b, "a", 1)


// let's make a negative test; a - b would be invalid
doubler((a, b) => +(a + b), "a", 1)
// "type number is not assignable to type string"
// "the expected type comes from the return type of this signature"