Package Exports
- @thi.ng/defmulti
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Readme
@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.
Installation
yarn add @thi.ng/defmultiUsage examples
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
// barSee /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: ${x}`); });
// 10 * (1 + 2 + 3) + 6
exec(["+", ["*", 10, ["+", 1, 2, 3]], 6]);
// 66True multiple arg dispatch
// interest rate calculator based on account type & balance thresholds
const apr = defmulti(
({type, balance}) => `${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: isadefmultiN
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.
const foo = defmultiN({
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: 2Authors
- Karsten Schmidt
License
© 2018 Karsten Schmidt // Apache Software License 2.0