This project is part of the @thi.ng/umbrella monorepo.
Dynamically extensible multiple dispatch via user supplied dispatch function, with minimal overhead.
yarn add @thi.ng/defmulti
defmulti returns a new multi-dispatch function using the provided
dispatcher function. The dispatcher 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.
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((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
// barconst 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: ${x}`); });
// 10 * (1 + 2 + 3) + 6
exec(["+", ["*", 10, ["+", 1, 2, 3]], 6]);
// 66// interest rate calculator based on account type & balance thresholds
const apr = defmulti(
({type, balance}) => `${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- Karsten Schmidt
© 2018 Karsten Schmidt // Apache Software License 2.0