libexpr: Add experimental pipe operator

The |> operator is a reverse function operator with low binding strength
to replace lib.pipe. Implements RFC 148, see the RFC text for more
details. Closes #438.

Change-Id: I21df66e8014e0d4dd9753dd038560a2b0b7fd805

1 files changed, 36 insertions, 0 deletions

diff --git a/tests/unit/libexpr/trivial.cc b/tests/unit/libexpr/trivial.cc
index 19b62aff8..c984657fd 100644
--- a/tests/unit/libexpr/trivial.cc
+++ b/tests/unit/libexpr/trivial.cc
@@ -210,4 +210,40 @@ namespace nix {
     TEST_F(TrivialExpressionTest, orCantBeUsed) {
         ASSERT_THROW(eval("let or = 1; in or"), Error);
     }
+
+    // pipes are gated behind an experimental feature flag
+    TEST_F(TrivialExpressionTest, pipeDisabled) {
+        ASSERT_THROW(eval("let add = l: r: l + r; in ''a'' |> add ''b''"), Error);
+        ASSERT_THROW(eval("let add = l: r: l + r; in ''a'' <| add ''b''"), Error);
+    }
+
+    TEST_F(TrivialExpressionTest, pipeRight) {
+        ExperimentalFeatureSettings mockXpSettings;
+        mockXpSettings.set("experimental-features", "pipe-operator");
+
+        auto v = eval("let add = l: r: l + r; in ''a'' |> add ''b''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("ba"));
+        v = eval("let add = l: r: l + r; in ''a'' |> add ''b'' |> add ''c''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("cba"));
+    }
+
+    TEST_F(TrivialExpressionTest, pipeLeft) {
+        ExperimentalFeatureSettings mockXpSettings;
+        mockXpSettings.set("experimental-features", "pipe-operator");
+
+        auto v = eval("let add = l: r: l + r; in add ''a'' <| ''b''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("ab"));
+        v = eval("let add = l: r: l + r; in add ''a'' <| add ''b'' <| ''c''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("abc"));
+    }
+
+    TEST_F(TrivialExpressionTest, pipeMixed) {
+        ExperimentalFeatureSettings mockXpSettings;
+        mockXpSettings.set("experimental-features", "pipe-operator");
+
+        auto v = eval("let add = l: r: l + r; in add ''a'' <| ''b'' |> add ''c''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("acb"));
+        v = eval("let add = l: r: l + r; in ''a'' |> add <| ''c''", true, mockXpSettings);
+        ASSERT_THAT(v, IsStringEq("ac"));
+    }
 } /* namespace nix */