libexpr: experimental pipe operators

NixOS · Jul 17, 2024 · 1012bb9 · 1012bb9
1 parent 621c23b
commit 1012bb9
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Showing 6 changed files with 96 additions and 8 deletions.
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
@@ -119,6 +119,12 @@ or          { return OR_KW; }
 \-\>        { return IMPL; }
 \/\/        { return UPDATE; }
 \+\+        { return CONCAT; }
+\<\|        { experimentalFeatureSettings.require(Xp::PipeOperators);
+              return APPLY;
+            }
+\|\>        { experimentalFeatureSettings.require(Xp::PipeOperators);
+              return APPLY_FLIPPED;
+            }
 
 {ID}        { yylval->id = {yytext, (size_t) yyleng}; return ID; }
 {INT}       { errno = 0;

diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
@@ -99,6 +99,14 @@ static void setDocPosition(const LexerState & lexerState, ExprLambda * lambda, P
     }
 }
 
+static Expr * makeCall(PosIdx pos, Expr * fn, Expr * arg) {
+    if (auto e2 = dynamic_cast<ExprCall *>(fn)) {
+        e2->args.push_back(arg);
+        return fn;
+    }
+    return new ExprCall(pos, fn, {arg});
+}
+
 
 %}
 
@@ -123,6 +131,7 @@ static void setDocPosition(const LexerState & lexerState, ExprLambda * lambda, P
 
 %type <e> start expr expr_function expr_if expr_op
 %type <e> expr_select expr_simple expr_app
+%type <e> expr_apply expr_apply_flipped
 %type <list> expr_list
 %type <attrs> binds
 %type <formals> formals
@@ -140,6 +149,7 @@ static void setDocPosition(const LexerState & lexerState, ExprLambda * lambda, P
 %token <path> PATH HPATH SPATH PATH_END
 %token <uri> URI
 %token IF THEN ELSE ASSERT WITH LET IN_KW REC INHERIT EQ NEQ AND OR IMPL OR_KW
+%token APPLY APPLY_FLIPPED /* <| and |> */
 %token DOLLAR_CURLY /* == ${ */
 %token IND_STRING_OPEN IND_STRING_CLOSE
 %token ELLIPSIS
@@ -206,9 +216,21 @@ expr_function
 
 expr_if
   : IF expr THEN expr ELSE expr { $$ = new ExprIf(CUR_POS, $2, $4, $6); }
+  | expr_apply
+  | expr_apply_flipped
   | expr_op
   ;
 
+expr_apply
+  : expr_op APPLY expr_apply { $$ = makeCall(state->at(@2), $1, $3); }
+  | expr_op APPLY expr_op    { $$ = makeCall(state->at(@2), $1, $3); }
+  ;
+
+expr_apply_flipped
+  : expr_apply_flipped APPLY_FLIPPED expr_op { $$ = makeCall(state->at(@2), $3, $1); }
+  | expr_op            APPLY_FLIPPED expr_op { $$ = makeCall(state->at(@2), $3, $1); }
+  ;
+
 expr_op
   : '!' expr_op %prec NOT { $$ = new ExprOpNot($2); }
   | '-' expr_op %prec NEGATE { $$ = new ExprCall(CUR_POS, new ExprVar(state->s.sub), {new ExprInt(0), $2}); }
@@ -233,13 +255,7 @@ expr_op
   ;
 
 expr_app
-  : expr_app expr_select {
-      if (auto e2 = dynamic_cast<ExprCall *>($1)) {
-          e2->args.push_back($2);
-          $$ = $1;
-      } else
-          $$ = new ExprCall(CUR_POS, $1, {$2});
-  }
+  : expr_app expr_select { $$ = makeCall(CUR_POS, $1, $2); }
   | expr_select
   ;
 

diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
@@ -24,7 +24,7 @@ struct ExperimentalFeatureDetails
  * feature, we either have no issue at all if few features are not added
  * at the end of the list, or a proper merge conflict if they are.
  */
-constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::VerifiedFetches);
+constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::PipeOperators);
 
 constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails = {{
     {
@@ -294,6 +294,14 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
         .trackingUrl = "https://github.com/NixOS/nix/milestone/48",
     },
+    {
+        .tag = Xp::PipeOperators,
+        .name = "pipe-operators",
+        .description = R"(
+            Enables |> and <| operators.
+        )",
+        .trackingUrl = "https://github.com/NixOS/rfcs/pull/148",
+    },
 }};
 
 static_assert(

diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
@@ -35,6 +35,7 @@ enum struct ExperimentalFeature
     ConfigurableImpureEnv,
     MountedSSHStore,
     VerifiedFetches,
+    PipeOperators,
 };
 
 /**

diff --git a/tests/unit/libexpr/main.cc b/tests/unit/libexpr/main.cc
@@ -34,6 +34,9 @@ int main (int argc, char **argv) {
     setEnv("_NIX_TEST_NO_SANDBOX", "1");
     #endif
 
+    // For pipe operator tests in trivial.cc
+    experimentalFeatureSettings.set("experimental-features", "pipe-operators");
+
     ::testing::InitGoogleTest(&argc, argv);
     return RUN_ALL_TESTS();
 }
diff --git a/tests/unit/libexpr/trivial.cc b/tests/unit/libexpr/trivial.cc
@@ -182,6 +182,60 @@ namespace nix {
         ASSERT_THAT(v, IsIntEq(15));
     }
 
+    TEST_F(TrivialExpressionTest, forwardPipe) {
+        auto v = eval("1 |> builtins.add 2 |> builtins.mul 3");
+        ASSERT_THAT(v, IsIntEq(9));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipe) {
+        auto v = eval("builtins.add 1 <| builtins.mul 2 <| 3");
+        ASSERT_THAT(v, IsIntEq(7));
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeEvaluationOrder) {
+        auto v = eval("1 |> null |> (x: 2)");
+        ASSERT_THAT(v, IsIntEq(2));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeEvaluationOrder) {
+        auto v = eval("(x: 1) <| null <| 2");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsDoNotAssociate) {
+        ASSERT_THROW(eval("(x: 1) <| 2 |> (x: 3)"), ParseError);
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsParensLeft) {
+        auto v = eval("((x: 1) <| 2) |> (x: 3)");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsParensRight) {
+        auto v = eval("(x: 1) <| (2 |> (x: 3))");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeLowestPrecedence) {
+        auto v = eval("false -> true |> (x: !x)");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeLowestPrecedence) {
+        auto v = eval("(x: !x) <| false -> true");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeStrongerThanElse) {
+        auto v = eval("if true then 1 else 2 |> 3");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeStrongerThanElse) {
+        auto v = eval("if true then 1 else 2 <| 3");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
     TEST_F(TrivialExpressionTest, bindOr) {
         auto v = eval("{ or = 1; }");
         ASSERT_THAT(v, IsAttrsOfSize(1));