followup nim-lang#16871 asyncjs.then: allow pipelining procs returnin…

…g futures

lib/js/asyncjs.nim

@@ -147,6 +147,7 @@ macro async*(arg: untyped): untyped =
       result.add generateJsasync(oneProc)
   else:
     result = generateJsasync(arg)
+  # echo result.repr # PRTEMP
 
 proc newPromise*[T](handler: proc(resolve: proc(response: T))): Future[T] {.importcpp: "(new Promise(#))".}
   ## A helper for wrapping callback-based functions
@@ -156,6 +157,8 @@ proc newPromise*(handler: proc(resolve: proc())): Future[void] {.importcpp: "(ne
   ## A helper for wrapping callback-based functions
   ## into promises and async procedures.
 
+# template `===>`(a)
+
 when defined(nimExperimentalAsyncjsThen):
   since (1, 5, 1):
     #[
@@ -180,6 +183,10 @@ when defined(nimExperimentalAsyncjsThen):
       ## See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
       asm "`result` = `future`.then(`onSuccess`, `onReject`)"
 
+    proc then*[T, T2](future: Future[T], onSuccess: proc(value: T): Future[T2], onReject: OnReject = nil): Future[T2] =
+      ## See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
+      asm "`result` = `future`.then(`onSuccess`, `onReject`)"
+
     proc then*[T](future: Future[T], onSuccess: proc(value: T), onReject: OnReject = nil): Future[void] =
       ## See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
       asm "`result` = `future`.then(`onSuccess`, `onReject`)"
@@ -216,4 +223,5 @@ when defined(nimExperimentalAsyncjsThen):
           assert  "foobar: 7" in $reason.message
         discard main()
 
+      # xxx add tests and examples for `then` with some `onReject` callback.
       asm "`result` = `future`.catch(`onReject`)"

tests/js/tasync.nim

@@ -50,6 +50,14 @@ proc fn(n: int): Future[int] {.async.} =
   else:
     return 10
 
+proc asyncFact(n: int): Future[int] {.async.} =
+  if n > 0: result = n * await asyncFact(n-1)
+  else: result = 1
+
+proc asyncIdentity(n: int): Future[int] {.async.} =
+  if n > 0: result = 1 + await asyncIdentity(n-1)
+  else: result = 0
+
 proc main() {.async.} =
   block: # then
     let x = await fn(4)
@@ -63,6 +71,17 @@ proc main() {.async.} =
     let x2 = await fn(4).then((a: int) => (discard)).then(() => 13)
     doAssert x2 == 13
 
+    let x4 = await asyncFact(3).then(asyncIdentity).then(asyncIdentity).then((a:int) => a * 7).then(asyncIdentity)
+    doAssert x4 == 3 * 2 * 7
+
+    when false:
+      # pending bug #17177
+      proc asyncIdentityNested(n: int): Future[int] {.async.} = return n
+      let x5 = await asyncFact(3).then(asyncIdentityNested)
+      doAssert x5 == 3 * 2
+      let x6 = await asyncFact(3).then((a:int) {.async.} => a * 11)
+      doAssert x6 == 3 * 2 * 11
+
   block: # catch
     var reason: Error
     await fn(6).then((a: int) => (witness.add $a)).catch((r: Error) => (reason = r))