From 4cea3467cb2347731d226e8d3ecee6a85b72d3ff Mon Sep 17 00:00:00 2001
From: Andreas Rumpf <rumpf_a@web.de>
Date: Thu, 6 Jun 2019 08:08:20 +0200
Subject: [PATCH] Updated Destructors, 2nd edition (rest)

---
 Destructors,-2nd-edition.rest | 45 ++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/Destructors,-2nd-edition.rest b/Destructors,-2nd-edition.rest
index 5e65cee..0992c93 100644
--- a/Destructors,-2nd-edition.rest
+++ b/Destructors,-2nd-edition.rest
@@ -2,7 +2,6 @@
 Nim Destructors and Move Semantics
 ==================================
 
-
 .. contents::
 
 
@@ -395,6 +394,50 @@ Cursor variables are commonly used in ``iterator`` implementations:
         yield it
 
 
+Lent type
+=========
+
+``proc p(x: sink T)`` means that the proc ``p`` takes ownership of ``x``.
+To eliminate even more creation/copy <-> destruction pairs, a proc's return
+type can be annotated as ``lent T``. This is useful for "getter" accessors
+that seek to allow an immutable view into a container.
+
+The ``sink`` and ``lent`` annotations allow us to remove most (if not all)
+superfluous copies and destructions.
+
+``lent T`` is like ``var T`` a hidden pointer. It is proven by the compiler
+that the pointer does not outlive its origin. No destructor call is injected
+for expressions of type ``lent T`` or of type ``var T``.
+
+
+.. code-block:: nim
+
+  type
+    Tree = object
+      kids: seq[Tree]
+
+  proc construct(kids: sink seq[Tree]): Tree =
+    result = Tree(kids: kids)
+    # converted into:
+    `=sink`(result.kids, kids)
+
+  proc `[]`*(x: Tree; i: int): lent Tree =
+    result = x.kids[i]
+    # borrows from 'x', this is transformed into:
+    result = addr x.kids[i]
+    # This means 'lent' is like 'var T' a hidden pointer.
+    # Unlike 'var' this cannot be used to mutate the object.
+
+  iterator children*(t: Tree): lent Tree =
+    for x in t.kids: yield x
+
+  proc main =
+    # everything turned into moves:
+    let t = construct(@[construct(@[]), construct(@[])])
+    echo t[0] # accessor does not copy the element!
+
+
+
 Owned refs
 ==========