Result

A function that can fail answers Result<Value, Failure>, whose cases are Ok and Fail. The postfix question mark unwraps an Ok or returns the Fail from the surrounding function.

There are no exceptions. A function that can fail says so in its result type, and the caller cannot ignore it. Result is an ordinary type with two cases, and the postfix ? operator is the only thing about it that is syntax.

Example#

use File from "std/fs"
use IoError from "std/fs"

fn firstLine(path: String): Result<String, IoError> {
  const text = File.readText(path)?
  const lines = text.lines()
  match lines.first() {
    Some(line) => line
    None => ""
  }
}

print firstLine("project.trb")

Syntax#

Result<Value, Failure>          the type
value                           the success, where a Result is expected: it becomes Ok(value)
Ok(value)      Ok value         the success case, written out
Fail(error)    Fail problem     the failure case
<expression>?                   unwrap an Ok, or return the Fail from the surrounding function
<expression> ?? <fallback>      the value, or the fallback (the fallback is lazy)

Ok and Fail are written bare because the prelude imports them from Result.

Rules#

Result<Value, Failure> has exactly two cases: case Ok(value: Value) and case Fail(error: Failure). It is declared in std/core and is not built into the language.
? unwraps an Ok and returns the Fail from the surrounding function. It is only legal in a function whose result is a Result or an Option, and at the top level of an entry file or a script.

? converts the error type through From. If the surrounding function answers Result<Value, AppError> and the expression answers Result<Value, IoError>, the conversion AppError.from(ioError) is applied. A case that wraps one value of a unique type gets that From generated, so nobody writes it:

use IoError from "std/fs"

type AppError {
  case Io(cause: IoError)
  case Startup(message: String)
}

fn start(text: String): Result<Int, AppError> {
  if text.isEmpty() {
    return Fail AppError.Startup("nothing to do")
  }
  text.byteLength()
}

print start("hello")

The success is the value itself. A value of exactly Value where a Result<Value, Failure> is expected - the end of a body, a return, an argument, a binding with an annotation - becomes Ok(value) on its own, the fifth coercion of Conversions. Ok is written only where that does not happen: where the Value is not decided yet, and in the payload of a nested Ok. A value that is an Option or a Result itself wraps into the Ok only where it is exactly the Value (Result<Int?, Failure> from an Int?). The failure is always written: return Fail problem.
```
fn percent(value: Int): Result<Int, String> {
  if value < 0 || value > 100 {
    return Fail "{value} is not a percentage"
  }
  value
}

print percent(42)
print percent(142)
```
A call in command position writes its argument without parentheses, which is why return Fail problem is the canonical form, and Ok Some(x) has parentheses on the inner call only.
?? is OrElse.orElse, and Option and Result both come with that trait. The right side is lazy, so it is evaluated only when it is needed, and it is checked against the Value rather than against the whole Result.
```
fn parseOr(text: String, fallback: Int): Int {
  Int.tryFrom(text) ?? fallback
}

print parseOr("12", 0)
print parseOr("nonsense", 0)
```
?. is not defined on Result. It is Option.map, and Option.flatMap when the member answers an Option, so it never produces a nested Option. To go from a Result to an Option, call ok().
Result shares the vocabulary of Option, Task and Iterate: map, flatMap, forEach, orElse, toList(). They mean the same thing everywhere, by convention rather than through higher-kinded types. map and flatMap work on the Ok side; mapError works on the Fail side.
isOk(), isError(), ok() and expect(message) are the rest of the surface. expect panics with the message and the error, and is for cases where a failure is a bug.
Precise error types are the norm for a library. A caller can only match on what a signature names. Result<Value, Error> - where Error is a trait from the prelude - is for the layers above, where the only thing left to do with a failure is to report it. Every error type that carries with Error fits into it through the ordinary coercion of a value to a trait-typed value.
A collection of results collects into a result of a collection. to<Result<List<Int>, ParseError>>() stops at the first Fail. That is what traverse or sequence is in a language with higher-kinded types, and it is an ordinary From<Iterate<...>> implementation rather than a language feature.
A top-level ? is not a panic. In an entry file or a script it prints error: <the error through Show> and exits with 1 (the caused by: lines of the chain are specified and not built yet, see Errors at the top level). A panic prints panic: <message> and the site, exits with 101, and runs nothing on the way out - no Close, no using cleanup.

What this is not#

Fail is not Err. The case is called Fail because Error is the name of the trait every error type implements, and because return Fail problem reads as what it does. The field keeps the name error, and so do isError and mapError, which are about the error the case carries.

fn checked(value: Int): Result<Int, String> {
  if value < 0 {
    return Fail "negative"
  }
  value
}

print checked(1)

Does not compile

fn checked(value: Int): Result<Int, String> {
  if value < 0 {
    return Err("negative")
  }
  value
}
// error: Cannot find `Err` here

? is not try and it is not a throw. It is an early return. Nothing is unwound, nothing is caught, and there is no handler anywhere. A function that does not want to return early matches on the Result instead.

A panic is not an exception. It cannot be caught, it aborts the process, and nothing runs while the program falls over, because a panic means the program has a bug and running more code in a broken program is how bugs get worse. The one exception is a sandboxed script, which the host can stop and report.

Result is not for absence. A value that may be missing is an Option<Value>, written Value?. A Result carries a reason.

Cases and match - how Ok and Fail are taken apart.
std/core - where Result, Option and Error are declared.
Read a file - the shortest complete use of ?.
Define an error type - one type with cases for a whole application.
Coming from Rust - Err against Fail, and what ? converts.

Example#

Syntax#

Rules#

What this is not#

Related#