Pipelines & Placeholders

Connect function calls with |> and trace how each result becomes the next input. Use the placeholder _ to choose which argument receives that input.

Install Topaz using First Program. For function declarations and calls, see Values and Functions.

Follow a value through three calls

Save this code as pipelines-placeholders.tpz in your working folder.

function add(value: int, amount: int) -> int {
    value + amount
}

function multiply(left: int, right: int) -> int {
    left * right
}

function equal(left: int, right: int) -> bool {
    left == right
}

let result = 5
    |> add(2)
    |> multiply(_, 3)
let repeated = result |> equal(_, _)

print("{result} {repeated}")

In that folder, check the file and run it.

topaz check pipelines-placeholders.tpz
topaz run pipelines-placeholders.tpz

The check succeeds with exit code 0. The program prints these results.

21 true

Trace the result

Pipelines run from left to right. Each stage evaluates its left side once, before anything on the right. Both placeholders in equal(_, _) receive that same saved value.

Choose where the input goes

Without a placeholder, the inserted argument comes before positional, spread, and named arguments. With placeholders, Topaz replaces them and adds no extra first argument.

A placeholder belongs to its nearest enclosing call. Several placeholders can receive the saved input. In a pattern, _ instead matches a value without binding a name.

Correct an argument position

In pipelines-placeholders.tpz, removing _ from multiply(_, 3) leaves multiply(3). The pipeline still inserts the previous result first and produces 21. Writing multiply(7, 3) there would supply three arguments after insertion and fail the check.

Put _ inside the receiving call's argument list. It cannot stand alone on the right or appear as a callee, member name, or field label.

Named arguments must follow positional and spread arguments. A right side that is neither a call, a function value, nor .field is a static error.

For an optional field, use ordinary optional chaining before the pipeline or inside a lambda. The form value |> ?.field is unsupported.

Handle errors and concurrency explicitly

Use postfix ? to propagate a Result and concurrent with a declared timeout for concurrent work. Pipelines do not automatically propagate errors, create partially applied functions, or start asynchronous work. User-defined pipe operators are unsupported.

See Operators & Expressions for precedence and Functions & Generics for function values. Collections & Comprehensions explains stages that produce collections.