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

4 files changed, 43 insertions, 0 deletions

diff --git a/src/libexpr/parser/grammar.hh b/src/libexpr/parser/grammar.hh
index 82df63bc5..2c5a3d1be 100644
--- a/src/libexpr/parser/grammar.hh
+++ b/src/libexpr/parser/grammar.hh
@@ -434,6 +434,8 @@ struct op {
     struct and_       : _op<TAO_PEGTL_STRING("&&"), 12> {};
     struct or_        : _op<TAO_PEGTL_STRING("||"), 13> {};
     struct implies    : _op<TAO_PEGTL_STRING("->"), 14, kind::rightAssoc> {};
+    struct pipe_right : _op<TAO_PEGTL_STRING("|>"), 15> {};
+    struct pipe_left  : _op<TAO_PEGTL_STRING("<|"), 16, kind::rightAssoc> {};
 };
 
 struct _expr {
@@ -521,6 +523,7 @@ struct _expr {
         app
     > {};
 
+    /* Order matters here. The order is the parsing order, not the precedence order: '<=' must be parsed before '<'. */
     struct _binary_operator : sor<
         operator_<op::implies>,
         operator_<op::update>,
@@ -529,6 +532,8 @@ struct _expr {
         operator_<op::minus>,
         operator_<op::mul>,
         operator_<op::div>,
+        operator_<op::pipe_right>,
+        operator_<op::pipe_left>,
         operator_<op::less_eq>,
         operator_<op::greater_eq>,
         operator_<op::less>,
@@ -649,6 +654,8 @@ struct operator_semantics {
             grammar::op::minus,
             grammar::op::mul,
             grammar::op::div,
+            grammar::op::pipe_right,
+            grammar::op::pipe_left,
             has_attr
         > op;
     };
diff --git a/src/libexpr/parser/parser.cc b/src/libexpr/parser/parser.cc
index 68aa3ddc5..6d496d141 100644
--- a/src/libexpr/parser/parser.cc
+++ b/src/libexpr/parser/parser.cc
@@ -113,6 +113,29 @@ struct ExprState
         return std::make_unique<ExprCall>(pos, std::make_unique<ExprVar>(fn), std::move(args));
     }
 
+    std::unique_ptr<Expr> pipe(PosIdx pos, State & state, bool flip = false)
+    {
+        if (!state.xpSettings.isEnabled(Xp::PipeOperator))
+            throw ParseError({
+                .msg = HintFmt("Pipe operator is disabled"),
+                .pos = state.positions[pos]
+            });
+
+        // Reverse the order compared to normal function application: arg |> fn
+        std::unique_ptr<Expr> fn, arg;
+        if (flip) {
+            fn = popExprOnly();
+            arg = popExprOnly();
+        } else {
+            arg = popExprOnly();
+            fn = popExprOnly();
+        }
+        std::vector<std::unique_ptr<Expr>> args{1};
+        args[0] = std::move(arg);
+
+        return std::make_unique<ExprCall>(pos, std::move(fn), std::move(args));
+    }
+
     std::unique_ptr<Expr> order(PosIdx pos, bool less, State & state)
     {
         return call(pos, state.s.lessThan, !less);
@@ -162,6 +185,8 @@ struct ExprState
                 [&] (Op::concat)      { return applyBinary<ExprOpConcatLists>(pos); },
                 [&] (has_attr & a)    { return applyUnary<ExprOpHasAttr>(std::move(a.path)); },
                 [&] (Op::unary_minus) { return negate(pos, state); },
+                [&] (Op::pipe_right)  { return pipe(pos, state, true); },
+                [&] (Op::pipe_left)   { return pipe(pos, state); },
             })(op)
         };
     }
diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
index 3a834293a..15a18c770 100644
--- a/src/libutil/experimental-features.cc
+++ b/src/libutil/experimental-features.cc
@@ -167,6 +167,16 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
     },
     {
+        .tag = Xp::PipeOperator,
+        .name = "pipe-operator",
+        .description = R"(
+            Enable new operators for function application to "pipe" arguments through a chain of functions similar to `lib.pipe`.
+            This implementation is based on Nix [RFC 148](https://github.com/NixOS/rfcs/pull/148).
+
+            Tracking issue: https://git.lix.systems/lix-project/lix/issues/438
+        )",
+    },
+    {
         .tag = Xp::FetchClosure,
         .name = "fetch-closure",
         .description = R"(
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
index 38889e7bc..121318d23 100644
--- a/src/libutil/experimental-features.hh
+++ b/src/libutil/experimental-features.hh
@@ -21,6 +21,7 @@ enum struct ExperimentalFeature
     NixCommand,
     RecursiveNix,
     NoUrlLiterals,
+    PipeOperator,
     FetchClosure,
     ReplFlake,
     AutoAllocateUids,