Fix: channels now support drain-after-close (close! marks closed, <! returns nil on exhausted closed channel, >! rejects on closed)

src/eval.nim

@@ -10,6 +10,7 @@ type
   Channel* = ref object
     buf: Deque[CljVal]
     capacity: int  # 0 = unbuffered
+    closed*: bool
 
 var channelRegistry* = initTable[string, Channel]()
 var channelCounter*: int64 = 0
@@ -1074,7 +1075,7 @@ proc evalList(items: seq[CljVal], env: Env): EvalResult =
       channelCounter += 1
       let id = "chan_" & $channelCounter
       let cap = if args.len > 0 and args[0].kind == ckInt: args[0].intVal.int else: 0
-      let ch = Channel(buf: initDeque[CljVal](), capacity: cap)
+      let ch = Channel(buf: initDeque[CljVal](), capacity: cap, closed: false)
       channelRegistry[id] = ch
       return EvalResult(ok: true, value: cljString(id))
 
@@ -1083,6 +1084,8 @@ proc evalList(items: seq[CljVal], env: Env): EvalResult =
       if args[0].kind != ckString or not channelRegistry.hasKey(args[0].strVal):
         return EvalResult(ok: false, error: "put!/<! requires a channel")
       let ch = channelRegistry[args[0].strVal]
+      if ch.closed:
+        return EvalResult(ok: false, error: "Channel is closed")
       if ch.capacity > 0 and ch.buf.len >= ch.capacity:
         return EvalResult(ok: false, error: "Channel buffer full")
       ch.buf.addLast(args[1])
@@ -1094,6 +1097,8 @@ proc evalList(items: seq[CljVal], env: Env): EvalResult =
         return EvalResult(ok: false, error: "take!/<! requires a channel")
       let ch = channelRegistry[args[0].strVal]
       if ch.buf.len == 0:
+        if ch.closed:
+          return EvalResult(ok: true, value: cljNil())
         return EvalResult(ok: true, value: cljNil())
       return EvalResult(ok: true, value: ch.buf.popFirst())
 
@@ -1101,7 +1106,8 @@ proc evalList(items: seq[CljVal], env: Env): EvalResult =
       numArgs(1)
       if args[0].kind != ckString or not channelRegistry.hasKey(args[0].strVal):
         return EvalResult(ok: false, error: "close! requires a channel")
-      channelRegistry.del(args[0].strVal)
+      let ch = channelRegistry[args[0].strVal]
+      ch.closed = true
       return EvalResult(ok: true, value: cljNil())
 
     of "go":