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libexpr: experimental pipe operators
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rhendric committed Jul 17, 2024
1 parent 621c23b commit 43be67e
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28 changes: 28 additions & 0 deletions doc/manual/rl-next/pipe-operators.md
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@@ -0,0 +1,28 @@
---
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][pipe 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*.

For example:

```
nix-repl> 1 |> builtins.add 2 |> builtins.mul 3
9
nix-repl> builtins.add 1 <| builtins.mul 2 <| 3
7
```

`<|` and `|>` are right and left associative, respectively, and have lower precedence than any other operator.
These properties may change in future releases.

See [the RFC](https://github.com/NixOS/rfcs/pull/148) for more examples and rationale.

[pipe operators]: @docroot@/language/operators.md#pipe-operators
37 changes: 36 additions & 1 deletion doc/manual/src/language/operators.md
Original file line number Diff line number Diff line change
Expand Up @@ -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**
Expand Down Expand Up @@ -182,3 +186,34 @@ Equivalent to `!`*b1* `||` *b2*.

[Logical implication]: #logical-implication

## Pipe operators

- *a* `|>` *b* is equivalent to *b* *a*
- *a* `<|` *b* is equivalent to *a* *b*

> **Example**
>
> ```
> nix-repl> 1 |> builtins.add 2 |> builtins.mul 3
> 9
>
> nix-repl> builtins.add 1 <| builtins.mul 2 <| 3
> 7
> ```
> **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
14 changes: 14 additions & 0 deletions src/libexpr/lexer.l
Original file line number Diff line number Diff line change
Expand Up @@ -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,
});
}

}

Expand Down Expand Up @@ -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;
Expand Down
30 changes: 23 additions & 7 deletions src/libexpr/parser.y
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Expand Up @@ -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});
}


%}

Expand All @@ -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
Expand All @@ -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
Expand Down Expand Up @@ -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}); }
Expand All @@ -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
;

Expand Down
10 changes: 9 additions & 1 deletion src/libutil/experimental-features.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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 = {{
{
Expand Down Expand Up @@ -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(
Expand Down
1 change: 1 addition & 0 deletions src/libutil/experimental-features.hh
Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@ enum struct ExperimentalFeature
ConfigurableImpureEnv,
MountedSSHStore,
VerifiedFetches,
PipeOperators,
};

/**
Expand Down
5 changes: 5 additions & 0 deletions tests/functional/lang/eval-fail-pipe-operators.err.exp
Original file line number Diff line number Diff line change
@@ -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|
1 change: 1 addition & 0 deletions tests/functional/lang/eval-fail-pipe-operators.nix
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@@ -0,0 +1 @@
1 |> 2
3 changes: 3 additions & 0 deletions tests/unit/libexpr/main.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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();
}
54 changes: 54 additions & 0 deletions tests/unit/libexpr/trivial.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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));
Expand Down

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