Guide to Typing ramda Functions

When passing an overloaded function into another function, typescript takes the last definition (see here). This means the most common definition should come last, eg

export function filter<T, P extends T>(pred: (val: T) => boolean): {
  (dict: Record<string, T>): Record<string, P>
  (list: readonly T[]): P[];
};

// if `(dict: Record<string, T>): Record<string, P>` was last,
// then `filter` would be expecting `Record<string, T>` as the argument and this function would fail
const pickFooAndFilter = pipe(prop('foo'), filter(isNotNil));

Additionally, the __: Placeholder argument must come first for overloads. It is important when that argument is just T, which is any. This is because __: Placeholder fulfills that generic, and you'll be in wrong overload when you pass it __.

// if you ordered it this way
export function assoc<T, K extends keyof U, U>(prop: K, val: T, obj: U): { /* ... */ }
export function assoc<K extends keyof U, U>(prop: K, __: Placeholder, obj: U): { /* ... */ }


// you'll end up with the wrong operation
const setFoo = assoc('foo', __, obj);
// would try to `obj['foo'] = __` instead of returning `(val: T) => O`