From d64ff983110fed10a7a3351ad156091cfaf3203b Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Wed, 6 Jun 2012 11:40:04 -0700
Subject: [PATCH] Revert "core: Remove swappable. Unused" due to test failures

This reverts commit ec5cbb4f5e951fcc51362c557b1968a57be36afc.
---
 src/libcore/core.rc      |  2 +
 src/libcore/swappable.rs | 98 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 src/libcore/swappable.rs

diff --git a/src/libcore/core.rc b/src/libcore/core.rc
index 6b4c7ad7d3c..84cebcceccd 100644
--- a/src/libcore/core.rc
+++ b/src/libcore/core.rc
@@ -43,6 +43,7 @@ export comm, task, future;
 export extfmt;
 export tuple;
 export to_str;
+export swappable;
 export dvec, dvec_iter;
 
 // NDM seems to be necessary for resolve to work
@@ -163,6 +164,7 @@ mod option_iter {
 }
 mod result;
 mod to_str;
+mod swappable;
 mod dvec;
 #[path="iter-trait"]
 mod dvec_iter {
diff --git a/src/libcore/swappable.rs b/src/libcore/swappable.rs
new file mode 100644
index 00000000000..5df9b2c4846
--- /dev/null
+++ b/src/libcore/swappable.rs
@@ -0,0 +1,98 @@
+export swappable;
+export unwrap;
+export methods;
+
+#[doc = "
+A value that may be swapped out temporarily while it is being processed
+and then replaced.  Swappables are most useful when working with unique
+values, which often cannot be mutated unless they are stored in the local
+stack frame to ensure memory safety.
+
+The type guarantees the invariant that the value is always \"swapped in\"
+except during the execution of the `swap()` and `with()` methods.
+"]
+type swappable<A> = {
+    mut o_t: option<A>
+};
+
+#[doc = "Create a swappable swapped in with a given initial value"]
+fn swappable<A>(+t: A) -> swappable<A> {
+    {mut o_t: some(t)}
+}
+
+#[doc = "Consumes a swappable and returns its contents without copying"]
+fn unwrap<A>(-s: swappable<A>) -> A {
+    let {o_t: o_t} <- s;
+    option::unwrap(o_t)
+}
+
+impl methods<A> for swappable<A> {
+    #[doc = "
+         Overwrites the contents of the swappable
+    "]
+    fn set(+a: A) {
+        self.o_t <- some(a);
+    }
+
+    #[doc = "
+         Invokes `f()` with the current value but replaces the
+         current value when complete.  Returns the result of `f()`.
+
+         Attempts to read or access the receiver while `f()` is executing
+         will fail dynamically.
+    "]
+    fn with<B>(f: fn(A) -> B) -> B {
+        let mut o_u = none;
+        self.swap { |t| o_u <- some(f(t)); t }
+        option::unwrap(o_u)
+    }
+
+    #[doc = "
+         Invokes `f()` with the current value and then replaces the
+         current value with the result of `f()`.
+
+         Attempts to read or access the receiver while `f()` is executing
+         will fail dynamically.
+    "]
+    fn swap(f: fn(-A) -> A) {
+        alt self.o_t {
+          none { fail "no value present---already swapped?"; }
+          some(_) {}
+        }
+
+        let mut o_t = none;
+        o_t <-> self.o_t;
+        self.o_t <- some(f(option::unwrap(o_t)));
+    }
+
+    #[doc = "True if there is a value present in this swappable"]
+    fn is_present() -> bool {
+        alt self.o_t {
+          none {false}
+          some(_) {true}
+        }
+    }
+
+    #[doc = "
+        Removes the value from the swappable.  Any further attempts
+        to use the swapabble without first invoking `set()` will fail.
+    "]
+    fn take() -> A {
+        alt self.o_t {
+          none { fail "swapped out"; }
+          some(_) {}
+        }
+
+        let mut o_t = none;
+        option::unwrap(o_t)
+    }
+}
+
+impl methods<A:copy> for swappable<A> {
+    #[doc = "
+        Copies out the contents of the swappable
+    "]
+    fn get() -> A {
+        self.o_t.get()
+    }
+}
\ No newline at end of file