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Pipe Operators and Lang

Overview

The pipe-operators experimental feature flag adds two pipe operators to the Nix language: |> and <|. They provide an alternative syntax for chaining function application—useful when a value should flow through several partial applications without nested parentheses. The flag is a language experiment: syntax and semantics may change until stabilisation, and evaluation fails unless you opt in explicitly.

Version stamp: As of the Nix 2.34.x stable reference manual (nix.dev/manual/nix/stable/ → 2.34; title 2.34.9), pipe-operators remains experimental (verified on Nix 2.34.8: manual pipeline examples evaluate to 9 and 7; without the flag, evaluation errors). This page covers the flag, how to enable it, and what the operators do at a high level. Full operator precedence and the rest of the language surface live in Operators and the Nix language domain.

Details

What the flag unlocks. With pipe-operators enabled, Nix accepts |> and <| in expressions. The Nix 2.34 operators chapter defines them as sugar for function application:

  • a |> b is equivalent to b a — the left-hand value is passed as the last argument to the callable on the right (left-to-right pipeline; left-associative, precedence 15).
  • a <| b is equivalent to a b — the right-hand value is passed as the last argument to the callable on the left (right-to-left pipeline; right-associative, precedence 15).

Both sit at the lowest precedence level among infix operators. Without the flag, expressions using them abort with an error that the experimental feature is disabled.

Experimental status. Like other flags in experimental features, pipe-operators is off by default. Language changes are exactly the sort of work guarded by flags: they can be reverted or altered in backwards-incompatible ways while maintainers gather feedback. Stabilisation is tracked on the pipe-operators tracking issue; see Tracking stabilization for the general lifecycle.

Not covered here. This leaf is about the pipe-operators flag and pipe syntax only. Other language-adjacent experimental features—such as fetch-tree (built-in), fetch-closure, or parse-toml-timestamps (TOML timestamp parsing in builtins.fromTOML)—are separate flags with their own pages or manual entries. For how flags fit together, see Feature flags overview.

Examples

Enable the flag in nix.conf (Nix 2.34.x):

extra-experimental-features = pipe-operators

Or pass it for a single evaluation:

nix-instantiate --extra-experimental-features pipe-operators -E '1 |> builtins.add 2 |> builtins.mul 3'

In nix repl / nix-instantiate with the flag enabled, the manual’s pipeline examples evaluate as follows (verified Nix 2.34.8 → 9 and 7):

# left-to-right: 1 |> add 2 |> mul 3  =>  mul 3 (add 2 1)  =>  9
1 |> builtins.add 2 |> builtins.mul 3

# right-to-left: add 1 <| mul 2 <| 3  =>  add 1 (mul 2 3)  =>  7
builtins.add 1 <| builtins.mul 2 <| 3

Without pipe-operators (Nix 2.34.8):

error: experimental Nix feature 'pipe-operators' is disabled; add '--extra-experimental-features pipe-operators' to enable it

References

See also