XO0177: syntax from another language (Go, Rust, TypeScript, Python, Swift, Kotlin); names the habit and the Xo form (decision 0087)

The code uses a form that is correct in Go, Rust, TypeScript, Python, Swift, or Kotlin but not in Xo. The message names the habit and the Xo form, and when the rewrite is unambiguous a machine fix applies it (xo fix). The parser then continues as if the Xo form had been written, so one habit gives one diagnostic. Common ones: func, fun, def, function are fn; x := v and Kotlin val are let (or var); Rust let mut is var; T: Hash + Eq is one constraint, T: Hash (Eq and Debug are automatic); Vec<T> and []T are List[T], map[K]V is Map[K, V]; var x or &x at a call site is just x (var belongs on the parameter); *p, &T, and *T have no Xo form (no pointers); c ? a : b and Python’s a if c else b are if c { a } else { b }; Kotlin ?: is ??; x!! and x! are x.or_fault("why"); switch/case and Kotlin when are match with pattern => body arms; elif is else if; and, or, not are &&, ||, !; x in xs is xs.contains(x); for i := 0; i < n; i++ is for i in 0..n; Go for k, v := range m is for (k, v) in m (the checker names the form for the type); for cond is while cond; loop and for {} are while {}; 0..<n is 0..n, Swift 0...n is 0..=n, Kotlin until is ..; x++ is x += 1; === is ==; ** is .pow(n); xs[a:b] is xs[a..b]; throw e and raise e are return Err(e); Swift try f() is f()? and throws is ! Error; try/catch has no Xo form (handle the Result); |x| e, (x) => e, and lambda x: e are fn(x) { e }; x as T is T(x); A::B is A.B; impl T { fn m(&self) } is fn (self: T) m() at top level; trait is interface; ): T and Go’s bare result type are -> T; Go (T, error) results are -> T ! Error, and return v, nil is return v; x, err := f() with if err != nil { return err } is let x = f()?; backtick templates and Python f-strings are "...${x}..."; /* */ is //. See decision 0087 for the policy.

Example

func add(a: Int, b: Int) -> Int {
  x := a + b
  x > 0 ? x : -x
}

Fix

fn add(a: Int, b: Int) -> Int {
  let x = a + b
  if x > 0 { x } else { -x }
}

Run xo explain XO0177 for this text in a terminal, or see section 10.6 of the specification.