feat(js): m08 stream (event streams as pure automata) with tests

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

js/solutions/stream.js

@@ -0,0 +1,71 @@
+// m08 stream: an event stream is a pure automaton (a Mealy machine).
+//   Stream<A> = { step: input => [outputs, nextStream] }
+// There is no hidden state: whatever a stream "remembers" lives in the next stream it returns.
+
+// Sources may return themselves as the next stream, since they remember nothing.
+export const keyDowns = { step: (i) => [i.tag === 'KeyDown' ? [i.key] : [], keyDowns] }
+
+export const keyUps = { step: (i) => [i.tag === 'KeyUp' ? [i.key] : [], keyUps] }
+
+// `last` is the time of the previous Time input. Multiples of ms in (last, t] fire, ascending.
+const intervalFrom = (ms, last) => ({
+  step: (i) => {
+    if (i.tag !== 'Time') return [[], intervalFrom(ms, last)]
+    const first = Math.floor(last / ms) + 1
+    const count = Math.max(0, Math.floor(i.t / ms) - first + 1)
+    return [Array.from({ length: count }, (_, n) => (first + n) * ms), intervalFrom(ms, i.t)]
+  },
+})
+
+export const interval = (ms) => intervalFrom(ms, 0)
+
+// Each combinator steps its inputs and wraps the *next* streams in the same combinator.
+export const map = (s, f) => ({
+  step: (i) => {
+    const [out, next] = s.step(i)
+    return [out.map(f), map(next, f)]
+  },
+})
+
+export const filter = (s, p) => ({
+  step: (i) => {
+    const [out, next] = s.step(i)
+    return [out.filter(p), filter(next, p)]
+  },
+})
+
+export const filterMap = (s, f) => filter(map(s, f), (b) => b !== null)
+
+export const merge = (a, b) => ({
+  step: (i) => {
+    const [outA, nextA] = a.step(i)
+    const [outB, nextB] = b.step(i)
+    return [[...outA, ...outB], merge(nextA, nextB)]
+  },
+})
+
+// The accumulator travels in the arguments of the next scan, not in a variable.
+export const scan = (s, f, z) => ({
+  step: (i) => {
+    const [out, next] = s.step(i)
+    const accs = out.reduce((xs, o) => [...xs, f(xs.length === 0 ? z : xs[xs.length - 1], o)], [])
+    return [accs, scan(next, f, accs.length === 0 ? z : accs[accs.length - 1])]
+  },
+})
+
+// Only the first step is special; afterwards we are just `next`.
+export const startWith = (s, v) => ({
+  step: (i) => {
+    const [out, next] = s.step(i)
+    return [[v, ...out], next]
+  },
+})
+
+export const run = (s, inputs) =>
+  inputs.reduce(
+    ([outs, stream], i) => {
+      const [out, next] = stream.step(i)
+      return [[...outs, ...out], next]
+    },
+    [[], s],
+  )[0]