72 lines
2.3 KiB
JavaScript
72 lines
2.3 KiB
JavaScript
// m08 stream: an event stream is a pure automaton (a Mealy machine).
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// Stream<A> = { step: input => [outputs, nextStream] }
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// There is no hidden state: whatever a stream "remembers" lives in the next stream it returns.
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// Sources may return themselves as the next stream, since they remember nothing.
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export const keyDowns = { step: (i) => [i.tag === 'KeyDown' ? [i.key] : [], keyDowns] }
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export const keyUps = { step: (i) => [i.tag === 'KeyUp' ? [i.key] : [], keyUps] }
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// `last` is the time of the previous Time input. Multiples of ms in (last, t] fire, ascending.
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const intervalFrom = (ms, last) => ({
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step: (i) => {
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if (i.tag !== 'Time') return [[], intervalFrom(ms, last)]
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const first = Math.floor(last / ms) + 1
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const count = Math.max(0, Math.floor(i.t / ms) - first + 1)
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return [Array.from({ length: count }, (_, n) => (first + n) * ms), intervalFrom(ms, i.t)]
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},
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})
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export const interval = (ms) => intervalFrom(ms, 0)
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// Each combinator steps its inputs and wraps the *next* streams in the same combinator.
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export const map = (s, f) => ({
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step: (i) => {
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const [out, next] = s.step(i)
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return [out.map(f), map(next, f)]
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},
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})
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export const filter = (s, p) => ({
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step: (i) => {
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const [out, next] = s.step(i)
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return [out.filter(p), filter(next, p)]
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},
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})
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export const filterMap = (s, f) => filter(map(s, f), (b) => b !== null)
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export const merge = (a, b) => ({
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step: (i) => {
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const [outA, nextA] = a.step(i)
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const [outB, nextB] = b.step(i)
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return [[...outA, ...outB], merge(nextA, nextB)]
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},
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})
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// The accumulator travels in the arguments of the next scan, not in a variable.
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export const scan = (s, f, z) => ({
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step: (i) => {
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const [out, next] = s.step(i)
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const accs = out.reduce((xs, o) => [...xs, f(xs.length === 0 ? z : xs[xs.length - 1], o)], [])
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return [accs, scan(next, f, accs.length === 0 ? z : accs[accs.length - 1])]
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},
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})
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// Only the first step is special; afterwards we are just `next`.
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export const startWith = (s, v) => ({
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step: (i) => {
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const [out, next] = s.step(i)
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return [[v, ...out], next]
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},
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})
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export const run = (s, inputs) =>
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inputs.reduce(
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([outs, stream], i) => {
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const [out, next] = stream.step(i)
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return [[...outs, ...out], next]
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},
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[[], s],
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)[0]
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