expression language: add nub_left

Originally, the expression lanuage only contained a function to
deduplicate a list, keeping only the right-most occurence. The
reason was that this is the order needed for linking: a library
providing an open symbol has to come on the command line after the
library using that symbol (and hence making it an open symbol).
However, by now use cases have emerged that require a topological
sorting where definition comes before use; also, when composing
the value of PATH from fragments, we usually want to keep the first
occurrence in order for it to take precedence. Therefore, also
add "nub_left" as built-in function, allowing a more condense (and
slightly more efficient) description in rules instead of the
revserse-nub_right-reverse pattern.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/buildtool/build_engine/expression/expression.test.cpp b/test/buildtool/build_engine/expression/expression.test.cpp
index 938bd63f..83d6a2fe 100644
--- a/test/buildtool/build_engine/expression/expression.test.cpp
+++ b/test/buildtool/build_engine/expression/expression.test.cpp
@@ -946,6 +946,19 @@ TEST_CASE("Expression Evaluation", "[expression]") {  // NOLINT
              )"_json));
     }
 
+    SECTION("nub_left expression") {
+        auto expr = Expression::FromJson(R"(
+             {"type": "nub_left"
+             , "$1": ["a", "b", "b", "a", "c", "b", "a"]
+             })"_json);
+        REQUIRE(expr);
+
+        auto result = expr.Evaluate(env, fcts);
+        REQUIRE(result);
+        REQUIRE(result->IsList());
+        CHECK(result == Expression::FromJson(R"(["a", "b", "c"])"_json));
+    }
+
     SECTION("change_ending") {
         auto expr = Expression::FromJson(R"(
             { "type": "change_ending"