stdlib: Add an either module

Like Haskell's, with left and right tags, various conversion functions

src/lib/either.rs

@@ -0,0 +1,64 @@
+import option;
+import option::some;
+import option::none;
+
+tag t[T, U] {
+    left(T);
+    right(U);
+}
+
+type operator[T, U] = fn(&T) -> U;
+
+fn either[T, U, V](&operator[T, V] f_left,
+                   &operator[U, V] f_right,
+                   &t[T, U] value) -> V {
+    alt (value) {
+        case (left(?l)) { f_left(l) }
+        case (right(?r)) { f_right(r) }
+    }
+}
+
+fn lefts[T, U](&vec[t[T, U]] eithers) -> vec[T] {
+    let vec[T] result = [];
+    for (t[T, U] elt in eithers) {
+        alt (elt) {
+            case (left(?l)) { result += [l] }
+            case (_) { /* fallthrough */ }
+        }
+    }
+    ret result;
+}
+
+fn rights[T, U](&vec[t[T, U]] eithers) -> vec[U] {
+    let vec[U] result = [];
+    for (t[T, U] elt in eithers) {
+        alt (elt) {
+            case (right(?r)) { result += [r] }
+            case (_) { /* fallthrough */ }
+        }
+    }
+    ret result;
+}
+
+fn partition[T, U](&vec[t[T, U]] eithers) -> tup(vec[T], vec[U]) {
+    let vec[T] lefts = [];
+    let vec[U] rights = [];
+    for (t[T, U] elt in eithers) {
+        alt (elt) {
+            case (left(?l)) { lefts += [l] }
+            case (right(?r)) { rights += [r] }
+        }
+    }
+    ret tup(lefts, rights);
+}
+
+//
+// Local Variables:
+// mode: rust
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C $RBUILD 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End:
+//

src/lib/std.rc

@@ -22,6 +22,7 @@ mod task;
 
 // Utility modules.
 
+mod either;
 mod option;
 mod util;
 

src/test/run-pass/lib-either.rs

@@ -0,0 +1,117 @@
+// xfail-stage0
+
+use std;
+import std::either::*;
+import std::vec::len;
+
+fn test_either_left() {
+  auto val = left(10);
+  fn f_left(&int x) -> bool { x == 10 }
+  fn f_right(&uint x) -> bool { false }
+  assert (either(f_left, f_right, val));
+}
+
+fn test_either_right() {
+  auto val = right(10u);
+  fn f_left(&int x) -> bool { false }
+  fn f_right(&uint x) -> bool { x == 10u }
+  assert (either(f_left, f_right, val));
+}
+
+fn test_lefts() {
+  auto input = [left(10),
+                right(11),
+                left(12),
+                right(13),
+                left(14)];
+  auto result = lefts(input);
+  assert (result == [10, 12, 14]);
+}
+
+fn test_lefts_none() {
+  let vec[t[int, int]] input = [right(10),
+                                right(10)];
+  auto result = lefts(input);
+  assert (len(result) == 0u);
+}
+
+fn test_lefts_empty() {
+  let vec[t[int, int]] input = [];
+  auto result = lefts(input);
+  assert (len(result) == 0u);
+}
+
+fn test_rights() {
+  auto input = [left(10),
+                right(11),
+                left(12),
+                right(13),
+                left(14)];
+  auto result = rights(input);
+  assert (result == [11, 13]);
+}
+
+fn test_rights_none() {
+  let vec[t[int, int]] input = [left(10),
+                                left(10)];
+  auto result = rights(input);
+  assert (len(result) == 0u);
+}
+
+fn test_rights_empty() {
+    let vec[t[int, int]] input = [];
+    auto result = rights(input);
+    assert (len(result) == 0u);
+}
+
+fn test_partition() {
+  auto input = [left(10),
+                right(11),
+                left(12),
+                right(13),
+                left(14)];
+  auto result = partition(input);
+  assert (result._0.(0) == 10);
+  assert (result._0.(1) == 12);
+  assert (result._0.(2) == 14);
+  assert (result._1.(0) == 11);
+  assert (result._1.(1) == 13);
+}
+
+fn test_partition_no_lefts() {
+  let vec[t[int, int]] input = [right(10),
+                                right(11)];
+  auto result = partition(input);
+  assert (len(result._0) == 0u);
+  assert (len(result._1) == 2u);
+}
+
+fn test_partition_no_rights() {
+  let vec[t[int, int]] input = [left(10),
+                                left(11)];
+  auto result = partition(input);
+  assert (len(result._0) == 2u);
+  assert (len(result._1) == 0u);
+}
+
+fn test_partition_empty() {
+  let vec[t[int, int]] input = [];
+  auto result = partition(input);
+  assert (len(result._0) == 0u);
+  assert (len(result._1) == 0u);
+}
+
+fn main() {
+  test_either_left();
+  test_either_right();
+  test_lefts();
+  test_lefts_none();
+  test_lefts_empty();
+  test_rights();
+  test_rights_none();
+  test_rights_empty();
+  test_partition();
+  test_partition_no_lefts();
+  test_partition_no_rights();
+  test_partition_empty();
+}