Forbidden & Deferred Forms

If topaz check rejects a familiar spelling, find its Topaz replacement below. For features without a direct replacement, choose an alternative that fits the result you need.

Run the supported forms

Install Topaz through First Program, then save this as forbidden-deferred.tpz.

TOPAZ

let mut count = 0
let values: Array<int> = [1, 2]
let transform: (int) -> int = value => value + 1

match values {
    case [head, ..tail] => {
        count = transform(head) + tail.length
    }
    case [] => {
        count = 0
    }
}

print("{count}")

Check and run it from the folder containing the file.

BASH

topaz check forbidden-deferred.tpz
topaz run forbidden-deferred.tpz

The check succeeds, and the program prints the following result.

Output

let mut allows count to change. The pattern binds head to 1 and tail to [2]. transform(head) returns 2, and adding the tail length gives 3. The type (int) -> int describes the function stored in transform.

Replace unsupported spellings

If you tried	Use instead
`mut let count = 0`	`let mut count = 0`
`count++` or `count--`	`count = count + 1` or `count = count - 1`
`[T]` as a type	`Array<T>`
`function(T) -> U` as a type	`(T) -> U`
`args: ...T`	`...args: T`
`[head, ...tail]`	`[head, ..tail]`
An anonymous `function (...) { ... }`	A lambda such as `value => value + 1`
Backticks or `${value}`	Double quotes and `{value}` interpolation
String indexing or `.length`	`text.scalars()` followed by Array operations

In forbidden-deferred.tpz, replacing let mut with mut let makes the check fail. Restore let mut on the first line, then check and run the file again.

For imports, name the members you need. Mark exported declarations with export where they are declared. Modules & Visibility shows the forms. use, string or template module paths, export lists, wildcard exports, re-exports, and export-site renaming are unsupported.

Choose an alternative for an unsupported feature

The features below are unavailable in the current language. These alternatives solve the stated tasks and may require changing the program's structure.

Task	Unsupported feature	Current choice
Inspect text	Grapheme-cluster APIs and direct string slices	Use `scalars()` when Unicode scalar boundaries fit the task
Build text or structured templates	HTML and custom template tags	Use strings or the built-in `p`, `r`, `sh`, `sql` tags for their stated purposes
Express generic requirements	Higher-rank constraints and general dynamic protocol dispatch	Use the supported named-function bounds in Functions & Generics
Describe recursive data	Recursive type aliases	Use supported enum declarations from Records & Nominal Data
Share declarations between modules	Package re-exports	Import members from the module that declares them
Expand elements	General iterable spread	Use Array spread in its supported positions, or iterate explicitly
Run work concurrently	`async` and `await`	Use `concurrent` when collecting named results with a timeout fits the task
Close resources	Asynchronous cleanup	Use synchronous `defer` or supported `using` forms
Handle an expected failure	Fault catching and `panic`, `throw`, `catch`	Return a `Result` for recoverable errors
Read an optional field in a pipeline	Optional-property pipe shorthand	Write an explicit expression using `?.`

A runtime fault stops execution. Changing its syntax cannot make it a recoverable Result. Prevent the invalid operation or use an API that returns an ordinary error value. See Null, Option, Result & Faults.

Rust forms such as &self, &mut self, Vec<T>, and use crate:: are not Topaz syntax. Use module-level impl Name, built-in collection types, explicit imports, and Topaz record updates.

Find examples in Syntax at a Glance. For operator corrections, continue with Operators & Expressions.