cps: experimental support for for-loops

For-loops are inlined using block statements, which messes with their
control flow due to #76.

Breaks a test in tzevv due to defer being rewritten into
try-finally, which doesn't work.

Co-authored-by: Andy Davidoff <github@andy.disruptek.com>

cps.nim

@@ -941,13 +941,21 @@ proc cpsXfrm(T: NimNode, n: NimNode): NimNode =
     result = copy n
   result = workaroundRewrites(result)
 
-macro cps*(T: typed, n: typed): untyped =
+macro cps2(T: typed, n: typed): untyped =
   # I hate doing stuff inside macros, call the proc to do the work
   when defined(nimdoc):
     result = n
   else:
     result = cpsXfrm(T, n)
 
+macro cps*(T: typed, n: typed): untyped =
+  case n.kind
+  of nnkProcDef:
+    result = getImplTransformed n.name
+    result.addPragma newColonExpr(bindSym"cps2", T)
+  else:
+    result = cpsXfrm(T, n)
+
 macro cpsMagic*(n: untyped): untyped =
   ## upgrade cps primitives to generate errors out of context
   ## and take continuations as input inside {.cps.} blocks

cps/spec.nim

@@ -324,7 +324,12 @@ proc normalizingRewrites*(n: NimNode): NimNode =
         if n.len == 2:
           n.add newEmptyNode()
         elif n[1].isEmpty:          # add explicit type symbol
-          n[1] = getTypeInst n[2]
+          if not n[2].isEmpty:
+            n[1] = getTypeInst n[2]
+          else:
+            # get the type from the symbol as the last resort.
+            # walkaround for #48.
+            n[1] = getTypeInst n[0]
         n[2] = normalizingRewrites n[2]
         result = n
 
@@ -439,6 +444,15 @@ proc normalizingRewrites*(n: NimNode): NimNode =
       else:
         discard
 
+    proc rewriteFastAsgn(n: NimNode): NimNode =
+      ## Rewrite nnkFastAsgn into nnkAsgn because sem don't like them
+      case n.kind
+      of nnkFastAsgn:
+        result = newNimNode(nnkAsgn, n)
+        for child in n.items:
+          result.add child
+      else: discard
+
     case n.kind
     of nnkIdentDefs:
       rewriteIdentDefs n
@@ -452,6 +466,8 @@ proc normalizingRewrites*(n: NimNode): NimNode =
       rewriteReturn n
     of CallNodes:
       rewriteHidden n
+    of nnkFastAsgn:
+      rewriteFastAsgn n
     else:
       nil
 

tests/taste.nim

@@ -998,19 +998,16 @@ suite "tasteful tests":
 
   block:
     ## for loops with continue, break and a split
-    when true:
-      skip"pending #48"
-    else:
-      r = 0
-      proc foo() {.cps: Cont.} =
-        inc r
-        for i in 0 .. 3:
-          noop()
-          if i == 0:
-            continue
-          if i > 2:
-            break
-          r.inc i
+    r = 0
+    proc foo() {.cps: Cont.} =
+      inc r
+      for i in 0 .. 3:
+        noop()
+        if i == 0:
+          continue
+        if i > 2:
+          break
+        r.inc i
 
-      trampoline foo()
-      check r == 4
+    trampoline foo()
+    check r == 4