feat(js): m07 io (effects as values) with trace interpreter and tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

js/solutions/io.js

@@ -0,0 +1,34 @@
+// m07 io: effects as values. An IO is a plain-data description of a program; nothing runs until interpreted.
+//   {tag:'Pure', value}      - done, with a value
+//   {tag:'Effect', op, k}    - perform `op`, then continue with `k(result)`
+
+export const pure = (value) => ({ tag: 'Pure', value })
+
+export const effect = (op) => ({ tag: 'Effect', op, k: pure })
+
+// Pure: apply f now. Effect: keep the op, and extend the continuation so f runs after k.
+// Nothing is performed here, we only build a bigger description.
+export const flatMap = (io, f) =>
+  io.tag === 'Pure'
+    ? f(io.value)
+    : { tag: 'Effect', op: io.op, k: (result) => flatMap(io.k(result), f) }
+
+export const map = (io, f) => flatMap(io, (a) => pure(f(a)))
+
+export const sequence = (ios) =>
+  ios.reduceRight((rest, io) => flatMap(io, () => rest), pure(undefined))
+
+export const draw = (shape) => effect({ tag: 'Draw', shape })
+
+export const randomSeed = effect({ tag: 'RandomSeed' })
+
+export const drawScene = (scene) => sequence(scene.map(draw))
+
+// Answer each op with a canned result, collecting the ops as we go (recursion, no mutation).
+const answer = (op, seed) => (op.tag === 'RandomSeed' ? seed : undefined)
+
+export const trace = (io, seed) => {
+  if (io.tag === 'Pure') return { result: io.value, ops: [] }
+  const rest = trace(io.k(answer(io.op, seed)), seed)
+  return { result: rest.result, ops: [io.op, ...rest.ops] }
+}