added system.onRaise to support a condition system

lib/system.nim

@@ -1941,6 +1941,7 @@ when not defined(EcmaScript) and not defined(NimrodVM):
       prev: PSafePoint # points to next safe point ON THE STACK
       status: int
       context: C_JmpBuf
+      raiseAction: proc (e: ref E_Base): bool {.closure.}
   
   when defined(initAllocator):
     initAllocator()
@@ -2040,6 +2041,14 @@ when not defined(EcmaScript) and not defined(NimrodVM):
       var e = getCurrentException()
       return if e == nil: "" else: e.msg
 
+    proc onRaise*(action: proc(e: ref E_Base): bool{.closure.}) =
+      ## can be used in a ``try`` statement to setup a Lisp-like
+      ## `condition system`:idx:\: This prevents the 'raise' statement to
+      ## raise an exception but instead calls ``action``.
+      ## If ``action`` returns false, the exception has been handled and
+      ## does not propagate further through the call stack.
+      if not isNil(excHandler): excHandler.raiseAction = action
+
   {.push stack_trace: off, profiler:off.}
   when defined(endb):
     include "system/debugger"

lib/system/excpt.nim

@@ -52,6 +52,7 @@ proc setFrame(s: PFrame) {.compilerRtl, inl.} =
   framePtr = s
 
 proc pushSafePoint(s: PSafePoint) {.compilerRtl, inl.} = 
+  s.raiseAction = nil
   s.prev = excHandler
   excHandler = s
 
@@ -198,8 +199,9 @@ proc raiseExceptionAux(e: ref E_Base) =
   if globalRaiseHook != nil:
     if not globalRaiseHook(e): return
   if excHandler != nil:
-    pushCurrentException(e)
-    c_longjmp(excHandler.context, 1)
+    if isNil(excHandler.raiseAction) or excHandler.raiseAction(e):
+      pushCurrentException(e)
+      c_longjmp(excHandler.context, 1)
   elif e[] of EOutOfMemory:
     writeToStdErr(e.name)
     quitOrDebug()