fixes #3995

tests/closure/tflatmap.nim

@@ -0,0 +1,24 @@
+
+# bug #3995
+
+import future
+
+type
+  RNG* = tuple[]
+  Rand*[A] = (RNG) -> (A, RNG)
+
+proc nextInt*(r: RNG): (int, RNG) =
+  (1, ())
+
+proc flatMap[A,B](f: Rand[A], g: A -> Rand[B]): Rand[B] =
+  (rng: RNG) => (
+    let (a, rng2) = f(rng);
+    let g1 = g(a);
+    g1(rng2)
+  )
+
+proc map[A,B](s: Rand[A], f: A -> B): Rand[B] =
+  let g: A -> Rand[B] = (a: A) => ((rng: RNG) => (f(a), rng))
+  flatMap(s, g)
+
+let f = nextInt.map(i => i - i mod 2)