Function: partialApply()

Binds the first argument to a function, returning a new function that takes the remaining arguments.

partialApply is curried. See curry.

Example

const concat = (a: string, b: string) => a + b;

const toUsername = partialApply("@", concat);

toUsername("elias"); // => "@elias"

Manual currying

Due to limitations of TypeScript, longlast's curry function does not support currying functions with type parameters. You can, however, use partialApply to create manually curried functions with type parameters, as in the following example:

// Two-argument signature
function filter<T>(
    callback: (elem: T) => boolean,
    array: T[],
): T[];

// One-argument signature
function filter<T>(
    callback: (elem: T) => boolean,
    _?: never,
): (array: T[]) => T[];

// Implementation
function filter(callback: any, array?: any): any {
    if (arguments.length === 1) {
        return partialApply(callback, filter);
    }

    return (array as any).filter(callback);
}

Note the second parameter in the "one-argument signature," _?: never. Its purpose is to prevent the function being called with a second argument of the wrong type.

A downside of this approach is that the implementation of the function is not typesafe, and you must write each call signature yourself.

Call Signature

partialApply<Arg, Rest, Return>(arg: Arg, f: (a: Arg, ...rest: Rest) => Return): PartiallyApplied<Rest, Return>

Defined in: pkg/partial-apply/src/index.ts:20

Type Parameters

Parameters

Returns

PartiallyApplied<Rest, Return>

Call Signature

partialApply<Arg>(arg: Arg, f?: undefined): <Rest, Return>(f: (arg: Arg, ...rest: Rest) => Return) => PartiallyApplied<Rest, Return>

Defined in: pkg/partial-apply/src/index.ts:25

Type Parameters

Parameters

Returns

<Rest, Return>(f: (arg: Arg, ...rest: Rest) => Return): PartiallyApplied<Rest, Return>

Type Parameters

Parameters

Returns

PartiallyApplied<Rest, Return>