Introduce template withValue() for tables.nim too.

lib/pure/collections/tables.nim

@@ -136,6 +136,51 @@ proc mget*[A, B](t: var Table[A, B], key: A): var B {.deprecated.} =
 
 proc getOrDefault*[A, B](t: Table[A, B], key: A): B = getOrDefaultImpl(t, key)
 
+template withValue*[A, B](t: var Table[A, B], key: A,
+                          value, body: untyped) =
+  ## retrieves the value at ``t[key]``. 
+  ## `value` can be modified in the scope of the ``withValue`` call.
+  ##
+  ## .. code-block:: nim
+  ##
+  ##   sharedTable.withValue(key, value) do:
+  ##     # block is executed only if ``key`` in ``t``
+  ##     value.name = "username" 
+  ##     value.uid = 1000
+  ##
+  mixin rawGet
+  var hc: Hash
+  var index = rawGet(t, key, hc)
+  let hasKey = index >= 0
+  if hasKey:
+    var value {.inject.} = addr(t.data[index].val)
+    body
+
+template withValue*[A, B](t: var Table[A, B], key: A,
+                          value, body1, body2: untyped) =
+  ## retrieves the value at ``t[key]``. 
+  ## `value` can be modified in the scope of the ``withValue`` call.
+  ## 
+  ## .. code-block:: nim
+  ##
+  ##   table.withValue(key, value) do:
+  ##     # block is executed only if ``key`` in ``t``
+  ##     value.name = "username" 
+  ##     value.uid = 1000
+  ##   do:
+  ##     # block is executed when ``key`` not in ``t``
+  ##     raise newException(KeyError, "Key not found")
+  ##
+  mixin rawGet
+  var hc: Hash
+  var index = rawGet(t, key, hc)
+  let hasKey = index >= 0
+  if hasKey:
+    var value {.inject.} = addr(t.data[index].val)
+    body1
+  else:
+    body2
+
 iterator allValues*[A, B](t: Table[A, B]; key: A): B =
   ## iterates over any value in the table `t` that belongs to the given `key`.
   var h: Hash = hash(key) and high(t.data)