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Appendix A: Essential Functions Support
In this appendix, you'll find some basic JavaScript implementations of various functions described in the book. Keep in mind that these implementations may not be the fastest or the most efficient implementation out there; they solely serve an educational purpose.
always
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// always :: a -> b -> a
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const always = curry((a, b) => a);
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compose
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// compose :: ((y -> z), (x -> y), ..., (a -> b)) -> a -> z
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const compose = (...fns) => (...args) => fns.reduceRight((res, fn) => [fn.call(null, ...res)], args)[0];
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curry
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// curry :: ((a, b, ...) -> c) -> a -> b -> ... -> c
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function curry(fn) {
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const arity = fn.length;
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return function $curry(...args) {
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if (args.length < arity) {
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return $curry.bind(null, ...args);
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}
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return fn.call(null, ...args);
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};
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}
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either
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// either :: (a -> c) -> (b -> c) -> Either a b -> c
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const either = curry((f, g, e) => {
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if (e.isLeft) {
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return f(e.$value);
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}
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return g(e.$value);
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});
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identity
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// identity :: x -> x
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const identity = x => x;
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inspect
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// inspect :: a -> String
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const inspect = (x) => {
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if (x && typeof x.inspect === 'function') {
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return x.inspect();
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}
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function inspectFn(f) {
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return f.name ? f.name : f.toString();
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}
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function inspectTerm(t) {
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switch (typeof t) {
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case 'string':
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return `'${t}'`;
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case 'object': {
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const ts = Object.keys(t).map(k => [k, inspect(t[k])]);
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return `{${ts.map(kv => kv.join(': ')).join(', ')}}`;
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}
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default:
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return String(t);
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}
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}
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function inspectArgs(args) {
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return Array.isArray(args) ? `[${args.map(inspect).join(', ')}]` : inspectTerm(args);
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}
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return (typeof x === 'function') ? inspectFn(x) : inspectArgs(x);
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};
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left
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// left :: a -> Either a b
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const left = a => new Left(a);
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liftA2
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// liftA2 :: (Applicative f) => (a1 -> a2 -> b) -> f a1 -> f a2 -> f b
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const liftA2 = curry((fn, a1, a2) => a1.map(fn).ap(a2));
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liftA3
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// liftA3 :: (Applicative f) => (a1 -> a2 -> a3 -> b) -> f a1 -> f a2 -> f a3 -> f b
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const liftA3 = curry((fn, a1, a2, a3) => a1.map(fn).ap(a2).ap(a3));
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maybe
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// maybe :: b -> (a -> b) -> Maybe a -> b
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const maybe = curry((v, f, m) => {
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if (m.isNothing) {
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return v;
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}
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return f(m.$value);
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});
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nothing
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// nothing :: Maybe a
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const nothing = Maybe.of(null);
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reject
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// reject :: a -> Task a b
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const reject = a => Task.rejected(a);
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Last modified 11mo ago