The Pipe and the Placeholder

How Janus took Elixir’s best idea and made it honest.

The Operator That Needs No Introduction

If you’ve written Elixir, you’ve used |>. The operator that made functional programming feel like reading a sentence: take this, do that, then that, then that.

Janus shares it. Identical token, identical semantics:

prices |> array.filter { |p| p > 100 }
       |> array.map { |p| p * tax_rate }
       |> array.sum

Law: a |> f(b, c) desugars to f(a, b, c). Left-associative. Honest. No magic.

Spec authority: SPEC-016 §2 (Grafting). Chained pipeline correctness landed 2026-01-25 — left-associative parsing was the bug, fixed at janus_parser.zig:2672.

Where Janus Goes Further Than Elixir

Elixir’s |> is locked to first-argument injection. That’s why Elixir devs end up writing awkward Enum.map(list, fn) signatures just to make the pipe work; the standard library is contorted around the operator.

Janus has the __ placeholder for arbitrary position:

text |> string.replace(__, from: " ", to: "_")
     |> http.post(__, to: url, cap: net_post)

Mechanism over policy. The pipe doesn’t dictate how you write your function signatures; you point at the slot.

And the Second Path

UFCS gives you the OO-flavored chain when that reads better:

path.read_file().parse().validate().process()

Both lower through the same desugar in QTJIR. Same graph, different ergonomics. Pick the one that makes the data flow visible at the call site.

The Philosophy

This is a Janus design pattern worth naming: don’t force the user’s hand to make the language happy.

Elixir’s pipe is beautiful, but it forces library authors to design APIs around the first-argument convention. That’s policy baked into mechanism. Janus separates them:

The pipe (|>) gives you the Elixir flow you already know
The placeholder (__) frees you from positional tyranny
UFCS gives you the method-chain feel when the data is the subject

Three paths to the same graph. The compiler doesn’t care which ergonomics you chose. That’s the point.

The best language features are the ones you don’t notice. |> is one of them. __ is the one you’ll miss when you go back.