libexpr: experimental pipe operators

doc/manual/rl-next/pipe-operators.md

@@ -0,0 +1,14 @@
+---
+synopsis: Add `pipe-operators` experimental feature
+prs:
+- 11131
+---
+
+This is a draft implementation of [RFC 0148](https://github.com/NixOS/rfcs/pull/148).
+
+The `pipe-operators` experimental feature adds `<|` and `|>` operators to the Nix language.
+*a* `|>` *b* is equivalent to the function application *b* *a*, and
+*a* `<|` *b* is equivalent to the function application *a* *b*.
+
+`<|` and `|>` are right and left associative, respectively, and have lower precedence than any other operator.
+These properties may change in future releases.

doc/manual/src/language/operators.md

@@ -26,13 +26,17 @@
 | Logical conjunction (`AND`)            | *bool* `&&` *bool*                         | left          | 12         |
 | Logical disjunction (`OR`)             | *bool* <code>\|\|</code> *bool*            | left          | 13         |
 | [Logical implication]                  | *bool* `->` *bool*                         | right         | 14         |
+| [Pipe operator] (experimental)         | *expr* `\|>` *func*                        | left          | 15         |
+| [Pipe operator] (experimental)         | *func* `<\|` *expr*                        | right         | 15         |
 
 [string]: ./types.md#type-string
 [path]: ./types.md#type-path
-[number]: ./types.md#type-float <!-- TODO(@rhendric, #10970): rationalize this -->
+[number]: ./types.md#type-float
 [list]: ./types.md#list
 [attribute set]: ./types.md#attribute-set
 
+<!-- TODO(@rhendric, #10970): ^ rationalize number -> int/float -->
+
 ## Attribute selection
 
 > **Syntax**
@@ -182,3 +186,24 @@ Equivalent to `!`*b1* `||` *b2*.
 
 [Logical implication]: #logical-implication
 
+## Pipe operators
+
+- *a* `|>` *b* is equivalent to *b* *a*
+- *a* `<|` *b* is equivalent to *a* *b*
+
+> **Warning**
+>
+> This syntax is part of an
+> [experimental feature](@docroot@/contributing/experimental-features.md)
+> and may change in future releases.
+>
+> To use this syntax, make sure the
+> [`pipe-operators` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-pipe-operators)
+> is enabled.
+> For example, include the following in [`nix.conf`](@docroot@/command-ref/conf-file.md):
+>
+> ```
+> extra-experimental-features = pipe-operators
+> ```
+
+[Pipe operator]: #pipe-operators

src/libexpr/lexer.l

@@ -67,6 +67,14 @@ static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)
     return {result, size_t(t - result)};
 }
 
+static void requireExperimentalFeature(const ExperimentalFeature & feature, const Pos & pos)
+{
+    if (!experimentalFeatureSettings.isEnabled(feature))
+        throw ParseError(ErrorInfo{
+            .msg = HintFmt("experimental Nix feature '%1%' is disabled; add '--extra-experimental-features %1%' to enable it", showExperimentalFeature(feature)),
+            .pos = pos,
+        });
+}
 
 }
 
@@ -119,6 +127,12 @@ or          { return OR_KW; }
 \-\>        { return IMPL; }
 \/\/        { return UPDATE; }
 \+\+        { return CONCAT; }
+\<\|        { requireExperimentalFeature(Xp::PipeOperators, state->positions[CUR_POS]);
+              return APPLY;
+            }
+\|\>        { requireExperimentalFeature(Xp::PipeOperators, state->positions[CUR_POS]);
+              return APPLY_FLIPPED;
+            }
 
 {ID}        { yylval->id = {yytext, (size_t) yyleng}; return ID; }
 {INT}       { errno = 0;

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
   ;
 

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"(
+            Add `|>` and `<|` operators to the Nix language.
+        )",
+        .trackingUrl = "https://github.com/NixOS/nix/milestone/55",
+    },
 }};
 
 static_assert(

src/libutil/experimental-features.hh

@@ -35,6 +35,7 @@ enum struct ExperimentalFeature
     ConfigurableImpureEnv,
     MountedSSHStore,
     VerifiedFetches,
+    PipeOperators,
 };
 
 /**

tests/functional/lang/eval-fail-pipe-operators.err.exp

@@ -0,0 +1,5 @@
+error: experimental Nix feature 'pipe-operators' is disabled; add '--extra-experimental-features pipe-operators' to enable it
+       at /pwd/lang/eval-fail-pipe-operators.nix:1:3:
+            1| 1 |> 2
+             |   ^
+            2|

tests/functional/lang/eval-fail-pipe-operators.nix

@@ -0,0 +1 @@
+1 |> 2

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();
 }

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));