defmulti

@thi.ng/defmulti

This project is part of the @thi.ng/umbrella monorepo.

About

Dynamically extensible multiple dispatch via user supplied dispatch function, with minimal overhead. Provides generics for type checking up to 8 args, but generally works with any number of arguments. Why "only" 8?

"If you have a procedure with ten parameters, you probably missed some."

-- Alan Perlis

Installation

yarn add @thi.ng/defmulti

Dependencies

• @thi.ng/api
• @thi.ng/errors

Usage examples

defmulti

defmulti returns a new multi-dispatch function using the provided dispatcher function. The dispatcher acts as a mapping function, can take any number of arguments and must produce a dispatch value (string, number or symbol) used to lookup an implementation. If found, the impl is called with the same args. If no matching implementation is available, attempts to dispatch to DEFAULT impl. If none is registered, an error is thrown.

defmulti provides generics for type checking up to 8 args (plus the return type) and the generics will also apply to all implementations. If more than 8 args are required, defmulti will fall back to an untyped varargs solution.

Implementations for different dispatch values can be added and removed dynamically by calling .add(id, fn) or .remove(id) on the returned function.

import { defmulti, DEFAULT } from "@thi.ng/defmulti";

const visit = defmulti<any, void>((x) => Object.prototype.toString.call(x));

// register implementations for different dispatch types
// each dispatch value can only be registered once
visit.add("[object Array]", (x) => x.forEach(visit));
visit.add("[object Object]", (x) => { for(let k in x) visit([k, x[k]]); });
// ignore null values
visit.add("[object Null]", (x) => { });
// DEFAULT matches all other dispatch values
visit.add(DEFAULT, (x) => console.log("visit", x.toString()));

// call like normal fn
visit([{a: 1, b: ["foo", "bar", null, 42]}])
// a
// 1
// b
// foo
// bar
// 42

See /test/index.ts for a variation of this example.

Dynamic dispatch: Simple S-expression interpreter

const exec = defmulti((x) => Array.isArray(x) ? x[0] : typeof x);
exec.add("+", ([_, ...args]) => args.reduce((acc, n) => acc + exec(n), 0));
exec.add("*", ([_, ...args]) => args.reduce((acc, n) => acc * exec(n), 1));
exec.add("number", (x) => x);
exec.add(DEFAULT, (x) => { throw new Error(`invalid expr: ${x}`); });

// 10 * (1 + 2 + 3) + 6
exec(["+", ["*", 10, ["+", 1, 2, 3]], 6]);
// 66

True multiple arg dispatch

// interest rate calculator based on account type & balance thresholds
const apr = defmulti(
    ({type, balance}) =>
        `${type}-${balance < 1e4 ? "low" : balance < 5e4 ? "med" : "high"}`
);

apr.add("current-low",  ({ balance }) => balance * 0.005);
apr.add("current-med",  ({ balance }) => balance * 0.01);
apr.add("current-high", ({ balance }) => balance * 0.01);
apr.add("savings-low",  ({ balance }) => balance * 0.01);
apr.add("savings-med",  ({ balance }) => balance * 0.025);
apr.add("savings-high", ({ balance }) => balance * 0.035);
apr.add(DEFAULT, (x) => { throw new Error(`invalid account type: ${x.type}`)});

apr({type: "current", balance: 5000});
// 25
apr({type: "current", balance: 10000});
// 100
apr({type: "savings", balance: 10000});
// 250
apr({type: "isa", balance: 10000});
// Error: invalid account type: isa

defmultiN

Returns a multi-dispatch function which delegates to one of the provided implementations, based on the arity (number of args) when the function is called. Internally uses defmulti, so new arities can be dynamically added (or removed) at a later time. defmultiN also registers a DEFAULT implementation which simply throws an IllegalArityError when invoked.

Note: Unlike defmulti no argument type checking is supported, however you can specify the return type for the generated function.

const foo = defmultiN<string>({
  0: () => "zero",
  1: (x) => `one: ${x}`,
  3: (x, y, z) => `three: ${x}, ${y}, ${z}`
});

foo();
// zero
foo(23);
// one: 23
foo(1, 2, 3);
// three: 1, 2, 3
foo(1, 2);
// Error: illegal arity: 2

Authors

• Karsten Schmidt

License

© 2018 Karsten Schmidt // Apache Software License 2.0