Add capture test

inputs/passing/capture.ts

@@ -0,0 +1,46 @@
+// test_output! [1,6,17]
+
+export default function () {
+  const poly = buildPoly([1, 2, 3]); // 1 + 2*x + 3*x^2
+
+  return [
+    poly(0),
+    poly(1),
+    poly(2),
+  ];
+}
+
+type Poly = (x: number) => number;
+
+function pow(n: number): Poly {
+  // Captures the number n
+  return (x: number) => x ** n;
+}
+
+function add(p: Poly, q: Poly): Poly {
+  // Captures the functions p and q
+  return (x) => p(x) + q(x);
+}
+
+function scale(c: number, p: Poly): Poly {
+  // Captures the number c and the function p
+  return (x) => c * p(x);
+}
+
+// Little endian:
+// a0*x^0 + a1*x^1 + a2*x^2 + ...
+function buildPoly(a: number[]): Poly {
+  let poly: Poly = () => 0;
+
+  for (let i = 0; i < a.length; i++) {
+    const term = scale(a[i], pow(i));
+    poly = add(poly, term);
+  }
+
+  return poly;
+
+  // Also works:
+  // return a
+  //   .map((c, i) => scale(c, pow(i)))
+  //   .reduce(add, () => 0);
+}