---

yaml
---
r: 16285
b: refs/heads/try
c: 0218418
h: refs/heads/master
i:
  16283: 880246b
v: v3

Author: catamorphism

[refs]

@@ -2,5 +2,5 @@
 refs/heads/master: 61b1875c16de39c166b0f4d54bba19f9c6777d1a
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: 4a81779abd786ff22d71434c6d9a5917ea4cdfff
-refs/heads/try: 704a5a8c681a7e4cb044da9f7b1118baf79b1ff6
+refs/heads/try: 0218418428a6d21656d5ccf247cca30e47255b97
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105

branches/try/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 {

branches/try/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()
+    }
+}

branches/try/src/test/run-pass/swappable-test.rs

@@ -0,0 +1,13 @@
+import swappable::{swappable, methods};
+
+fn main() {
+    let d = swappable(3);
+    assert d.get() == 3;
+    d.set(4);
+    assert d.get() == 4;
+    d.swap { |i| i + 1 };
+    assert d.get() == 5;
+    assert d.with { |i| i + 1 } == 6;
+    assert d.get() == 5;
+    assert swappable::unwrap(d) == 5;
+}