Err module (Err)

Start here

Err is the usual way to return "something went wrong" from APIs that can fail. You bring Err. Many library calls return a pair (for example a value and an Err::Result, or a bool and an Err::Result). Check the error first with isErr() on the result or Err::isErr(err) before you trust the "success" part.

• Err::Result — holds a message, a small code (see Err::CODE), and optional details.
• Err::CODE — named constants for stable error kinds.

A tiny program

bring Err
print(Err::CODE.NOT_FOUND)

Output:

1

Pattern with file or other modules — the second (or last) return is often the Result:

bring file
bring Err

var content: str, err: Err::Result = file::readText("missing.txt")
if err.isErr() {
  println(err.toString())
} else {
  println(content)
}

Complete error codes

Constant	Integer value	Meaning
Err::CODE.OK	0	No error / success
Err::CODE.NOT_FOUND	1	Resource not found
Err::CODE.FAILED	2	General failure
Err::CODE.INVALID_ARGUMENT	3	Bad input

Building and inspecting errors

Constructors and helpers include an init with message, optional code, and optional details; and methods such as:

• getMessage(), getCode(), getDetails()
• withMessage(extraMessage) — returns a new Err::Result with the extra message appended to the original message (joined with ->). The original code and details are preserved.
• withDetails(details) — returns a new Err::Result with the details field set to the given string. The original message and code are preserved.
• withCode(code) — returns a new Err::Result with the code replaced. The original message and details are preserved.
• hasDetails() — returns bool, true if details is non-empty.
• isOk() / isErr() — check success or failure.
• clone() — returns a new Err::Result with all fields copied.
• toString() — a single human-readable line for logging or println.
• clearDetails() — returns a new Err::Result with details cleared to "".

Free functions:

• Err::ok(message) — returns a success Err::Result (code OK).
• Err::error(message, code, details) — returns a failure Err::Result.
• Err::isOk(result) / Err::isErr(result) — static checks on any Err::Result.

Longer example:

bring Err

fun handle_error() -> nthg {
var err1: Err::Result = new Err::Result("File not found", Err::CODE.NOT_FOUND)
print(err1.getMessage())

var err2: Err::Result = err1.withMessage("Please check the path")
print(err2.toString())

var err3: Err::Result = err2.withDetails("The file might have been deleted")
print(err3.toString())

var err4: Err::Result = new Err::Result("Invalid argument", Err::CODE.INVALID_ARGUMENT)
print(err4.toString())
}

handle_error()

Output:

Error: 
File not foundError: File not found -> Please check the pathError: File not found -> Please check the path (Details: The file might have been deleted)Error: Invalid argument

More detail: ErrorCode and Err::Result

Err::CODE is a shared vocabulary between your code and the standard library. Err::Result is designed so you can thread messages and details through calls without a separate exception system—Flow-Wing's model is explicit return values, not try/catch.

Source & tests (if you have the repository)

What	Where
Module source (reference)	fw-modules/error_module/Err-module.fg.
Integration-style fixtures	tests/fixtures/LatestTests/ErrModuleTests/.

Cloning the repo is optional for day-to-day use of Err.