Try

In Gleam if a function can either succeed or fail then it normally will return the Result type. With Result, a successful return value is wrapped in an Ok record, and an error value is wrapped in an Error record.

// parse_int(String) -> Result(Int, String)

parse_int("123") // -> Ok(123)
parse_int("erl") // -> Error("expected a number, got `erl`")

When a function returns a Result we can pattern match on it to handle success and failure:

case parse_int("123") {
  Error(e) -> io.println("That wasn't an Int")
  Ok(i) -> io.println("We parsed the Int")
}

This is such a common pattern in Gleam that the try syntax exists to make it more concise.

try int_a = parse_int(a)
try int_b = parse_int(b)
try int_c = parse_int(c)
Ok(int_a + int_b + int_c)

When a variable is declared using try Gleam checks to see whether the value is an Error or an Ok record. If it's an Ok then the inner value is assigned to the variable:

try x = Ok(1)
Ok(x + 1)
// -> Ok(2)

If it's an Error then the Error is returned immediately:

try x = Error("failure")
Ok(x + 1)
// -> Error("failure")