stand-alone except and finally blocks

This is equivalent to the on-error and at-scope-exit operators in other languages

compiler/parser.nim

@@ -998,7 +998,14 @@ proc parseTry(p: var TParser): PNode =
     addSon(result, b)
     if b.kind == nkFinally: break 
   if b == nil: parMessage(p, errTokenExpected, "except")
-  
+
+proc parseExceptBlock(p: var TParser, kind: TNodeKind): PNode =
+  result = newNodeP(kind, p)
+  getTok(p)
+  eat(p, tkColon)
+  skipComment(p, result)
+  addSon(result, parseStmt(p))
+
 proc parseFor(p: var TParser): PNode = 
   result = newNodeP(nkForStmt, p)
   getTok(p)
@@ -1393,6 +1400,8 @@ proc complexOrSimpleStmt(p: var TParser): PNode =
   of tkWhile: result = parseWhile(p)
   of tkCase: result = parseCase(p)
   of tkTry: result = parseTry(p)
+  of tkFinally: result = parseExceptBlock(p, nkFinally)
+  of tkExcept: result = parseExceptBlock(p, nkExceptBranch)
   of tkFor: result = parseFor(p)
   of tkBlock: result = parseBlock(p)
   of tkStatic: result = parseStatic(p)

compiler/semstmts.nim

@@ -871,13 +871,33 @@ proc SemStmt(c: PContext, n: PNode): PNode =
   of nkBlockStmt: result = semBlock(c, n)
   of nkStmtList: 
     var length = sonsLen(n)
-    for i in countup(0, length - 1): 
-      n.sons[i] = semStmt(c, n.sons[i])
-      if n.sons[i].kind in LastBlockStmts: 
-        for j in countup(i + 1, length - 1): 
-          case n.sons[j].kind
-          of nkPragma, nkCommentStmt, nkNilLit, nkEmpty: nil
-          else: localError(n.sons[j].info, errStmtInvalidAfterReturn)
+    for i in countup(0, length - 1):
+      case n.sons[i].kind
+      of nkFinally, nkExceptBranch:
+        # stand-alone finally and except blocks are
+        # transformed into regular try blocks:
+        #
+        # var f = fopen("somefile") | var f = fopen("somefile")
+        # finally: fcsole(f)        | try:
+        # ...                       |   ...
+        #                           | finally:
+        #                           |   fclose(f)
+        var tryStmt = newNodeI(nkTryStmt, n.sons[i].info)
+        var body = newNodeI(nkStmtList, n.sons[i].info)
+        if i < n.sonsLen - 1:
+          body.sons = n.sons[(i+1)..(-1)]
+        tryStmt.addSon(body)
+        tryStmt.addSon(n.sons[i])
+        n.sons[i] = semTry(c, tryStmt)
+        n.sons.setLen(i+1)
+        return
+      else:
+        n.sons[i] = semStmt(c, n.sons[i])
+        if n.sons[i].kind in LastBlockStmts: 
+          for j in countup(i + 1, length - 1): 
+            case n.sons[j].kind
+            of nkPragma, nkCommentStmt, nkNilLit, nkEmpty: nil
+            else: localError(n.sons[j].info, errStmtInvalidAfterReturn)
   of nkRaiseStmt: result = semRaise(c, n)
   of nkVarSection: result = semVarOrLet(c, n, skVar)
   of nkLetSection: result = semVarOrLet(c, n, skLet)