forked from lix-project/lix
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
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@ -103,6 +103,11 @@ midnightveil:
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ncfavier:
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ncfavier:
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github: ncfavier
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github: ncfavier
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piegames:
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display_name: piegames
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forgejo: piegames
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github: piegamesde
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puck:
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puck:
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display_name: puck
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display_name: puck
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forgejo: puck
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forgejo: puck
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10
doc/manual/rl-next/pipe-operator.md
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10
doc/manual/rl-next/pipe-operator.md
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@ -0,0 +1,10 @@
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---
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synopsis: Pipe operator `|>` (experimental)
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issues: [fj#438]
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cls: [1654]
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category: Features
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credits: [piegames, horrors]
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---
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Implementation of the pipe operator (`|>`) in the language as described in [RFC 148](https://github.com/NixOS/rfcs/pull/148).
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The feature is still marked experimental, enable `--extra-experimental-features pipe-operator` to use it.
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@ -26,6 +26,8 @@
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| Logical conjunction (`AND`) | *bool* `&&` *bool* | left | 12 |
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| Logical conjunction (`AND`) | *bool* `&&` *bool* | left | 12 |
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| Logical disjunction (`OR`) | *bool* <code>\|\|</code> *bool* | left | 13 |
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| Logical disjunction (`OR`) | *bool* <code>\|\|</code> *bool* | left | 13 |
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| [Logical implication] | *bool* `->` *bool* | none | 14 |
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| [Logical implication] | *bool* `->` *bool* | none | 14 |
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| \[Experimental\] [Function piping] | *expr* |> *func* | left | 15 |
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| \[Experimental\] [Function piping] | *expr* <| *func* | right | 16 |
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[string]: ./values.md#type-string
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[string]: ./values.md#type-string
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[path]: ./values.md#type-path
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[path]: ./values.md#type-path
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@ -215,3 +217,33 @@ nix-repl> let f = x: 1; s = { func = f; }; in [ (f == f) (s == s) ]
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Equivalent to `!`*b1* `||` *b2*.
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Equivalent to `!`*b1* `||` *b2*.
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[Logical implication]: #logical-implication
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[Logical implication]: #logical-implication
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## \[Experimental\] Function piping
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*This language feature is still experimental and may change at any time. Enable `--extra-experimental-features pipe-operator` to use it.*
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Pipes are a dedicated operator for function application, but with reverse order and a lower binding strength.
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This allows you to chain function calls together in way that is more natural to read and requires less parentheses.
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`a |> f b |> g` is equivalent to `g (f b a)`.
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`g <| f b <| a` is equivalent to `g (f b a)`.
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Example code snippet:
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```nix
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defaultPrefsFile = defaultPrefs
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|> lib.mapAttrsToList (
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key: value: ''
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// ${value.reason}
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pref("${key}", ${builtins.toJSON value.value});
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''
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)
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|> lib.concatStringsSep "\n"
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|> pkgs.writeText "nixos-default-prefs.js";
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```
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Note how `mapAttrsToList` is called with two arguments (the lambda and `defaultPrefs`),
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but moving the last argument in front of the rest improves the reading flow.
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This is common for functions with long first argument, including all `map`-like functions.
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[Function piping]: #experimental-function-piping
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@ -434,6 +434,8 @@ struct op {
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struct and_ : _op<TAO_PEGTL_STRING("&&"), 12> {};
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struct and_ : _op<TAO_PEGTL_STRING("&&"), 12> {};
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struct or_ : _op<TAO_PEGTL_STRING("||"), 13> {};
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struct or_ : _op<TAO_PEGTL_STRING("||"), 13> {};
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struct implies : _op<TAO_PEGTL_STRING("->"), 14, kind::rightAssoc> {};
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struct implies : _op<TAO_PEGTL_STRING("->"), 14, kind::rightAssoc> {};
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struct pipe_right : _op<TAO_PEGTL_STRING("|>"), 15> {};
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struct pipe_left : _op<TAO_PEGTL_STRING("<|"), 16, kind::rightAssoc> {};
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};
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};
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struct _expr {
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struct _expr {
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@ -521,6 +523,7 @@ struct _expr {
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app
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app
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> {};
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> {};
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/* Order matters here. The order is the parsing order, not the precedence order: '<=' must be parsed before '<'. */
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struct _binary_operator : sor<
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struct _binary_operator : sor<
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operator_<op::implies>,
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operator_<op::implies>,
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operator_<op::update>,
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operator_<op::update>,
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@ -529,6 +532,8 @@ struct _expr {
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operator_<op::minus>,
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operator_<op::minus>,
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operator_<op::mul>,
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operator_<op::mul>,
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operator_<op::div>,
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operator_<op::div>,
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operator_<op::pipe_right>,
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operator_<op::pipe_left>,
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operator_<op::less_eq>,
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operator_<op::less_eq>,
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operator_<op::greater_eq>,
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operator_<op::greater_eq>,
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operator_<op::less>,
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operator_<op::less>,
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@ -649,6 +654,8 @@ struct operator_semantics {
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grammar::op::minus,
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grammar::op::minus,
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grammar::op::mul,
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grammar::op::mul,
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grammar::op::div,
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grammar::op::div,
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grammar::op::pipe_right,
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grammar::op::pipe_left,
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has_attr
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has_attr
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> op;
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> op;
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};
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};
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@ -113,6 +113,29 @@ struct ExprState
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return std::make_unique<ExprCall>(pos, std::make_unique<ExprVar>(fn), std::move(args));
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return std::make_unique<ExprCall>(pos, std::make_unique<ExprVar>(fn), std::move(args));
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}
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}
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std::unique_ptr<Expr> pipe(PosIdx pos, State & state, bool flip = false)
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{
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if (!state.xpSettings.isEnabled(Xp::PipeOperator))
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throw ParseError({
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.msg = HintFmt("Pipe operator is disabled"),
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.pos = state.positions[pos]
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});
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// Reverse the order compared to normal function application: arg |> fn
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std::unique_ptr<Expr> fn, arg;
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if (flip) {
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fn = popExprOnly();
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arg = popExprOnly();
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} else {
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arg = popExprOnly();
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fn = popExprOnly();
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}
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std::vector<std::unique_ptr<Expr>> args{1};
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args[0] = std::move(arg);
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return std::make_unique<ExprCall>(pos, std::move(fn), std::move(args));
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}
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std::unique_ptr<Expr> order(PosIdx pos, bool less, State & state)
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std::unique_ptr<Expr> order(PosIdx pos, bool less, State & state)
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{
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{
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return call(pos, state.s.lessThan, !less);
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return call(pos, state.s.lessThan, !less);
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@ -162,6 +185,8 @@ struct ExprState
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[&] (Op::concat) { return applyBinary<ExprOpConcatLists>(pos); },
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[&] (Op::concat) { return applyBinary<ExprOpConcatLists>(pos); },
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[&] (has_attr & a) { return applyUnary<ExprOpHasAttr>(std::move(a.path)); },
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[&] (has_attr & a) { return applyUnary<ExprOpHasAttr>(std::move(a.path)); },
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[&] (Op::unary_minus) { return negate(pos, state); },
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[&] (Op::unary_minus) { return negate(pos, state); },
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[&] (Op::pipe_right) { return pipe(pos, state, true); },
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[&] (Op::pipe_left) { return pipe(pos, state); },
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})(op)
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})(op)
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};
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};
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}
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}
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@ -166,6 +166,16 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
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may confuse external tooling.
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may confuse external tooling.
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)",
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)",
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},
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},
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{
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.tag = Xp::PipeOperator,
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.name = "pipe-operator",
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.description = R"(
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Enable new operators for function application to "pipe" arguments through a chain of functions similar to `lib.pipe`.
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This implementation is based on Nix [RFC 148](https://github.com/NixOS/rfcs/pull/148).
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Tracking issue: https://git.lix.systems/lix-project/lix/issues/438
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)",
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},
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{
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{
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.tag = Xp::FetchClosure,
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.tag = Xp::FetchClosure,
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.name = "fetch-closure",
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.name = "fetch-closure",
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@ -21,6 +21,7 @@ enum struct ExperimentalFeature
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NixCommand,
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NixCommand,
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RecursiveNix,
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RecursiveNix,
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NoUrlLiterals,
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NoUrlLiterals,
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PipeOperator,
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FetchClosure,
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FetchClosure,
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ReplFlake,
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ReplFlake,
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AutoAllocateUids,
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AutoAllocateUids,
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@ -210,4 +210,40 @@ namespace nix {
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TEST_F(TrivialExpressionTest, orCantBeUsed) {
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TEST_F(TrivialExpressionTest, orCantBeUsed) {
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ASSERT_THROW(eval("let or = 1; in or"), Error);
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ASSERT_THROW(eval("let or = 1; in or"), Error);
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}
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}
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// pipes are gated behind an experimental feature flag
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TEST_F(TrivialExpressionTest, pipeDisabled) {
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ASSERT_THROW(eval("let add = l: r: l + r; in ''a'' |> add ''b''"), Error);
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ASSERT_THROW(eval("let add = l: r: l + r; in ''a'' <| add ''b''"), Error);
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}
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TEST_F(TrivialExpressionTest, pipeRight) {
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ExperimentalFeatureSettings mockXpSettings;
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mockXpSettings.set("experimental-features", "pipe-operator");
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auto v = eval("let add = l: r: l + r; in ''a'' |> add ''b''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("ba"));
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v = eval("let add = l: r: l + r; in ''a'' |> add ''b'' |> add ''c''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("cba"));
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}
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TEST_F(TrivialExpressionTest, pipeLeft) {
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ExperimentalFeatureSettings mockXpSettings;
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mockXpSettings.set("experimental-features", "pipe-operator");
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auto v = eval("let add = l: r: l + r; in add ''a'' <| ''b''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("ab"));
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v = eval("let add = l: r: l + r; in add ''a'' <| add ''b'' <| ''c''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("abc"));
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}
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TEST_F(TrivialExpressionTest, pipeMixed) {
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ExperimentalFeatureSettings mockXpSettings;
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mockXpSettings.set("experimental-features", "pipe-operator");
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auto v = eval("let add = l: r: l + r; in add ''a'' <| ''b'' |> add ''c''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("acb"));
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v = eval("let add = l: r: l + r; in ''a'' |> add <| ''c''", true, mockXpSettings);
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ASSERT_THAT(v, IsStringEq("ac"));
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}
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} /* namespace nix */
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} /* namespace nix */
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