NoReturn

Some expressions won't return to the current scope and therefore have no return type. This is expressed as the special return type NoReturn.

Typical examples for non-returning methods and keywords are return, exit, raise, next, and break.

This is for example useful for deconstructing union types:

string = STDIN.gets
typeof(string)                        # => String?
typeof(raise "Empty input")           # => NoReturn
typeof(string || raise "Empty input") # => String

The compiler recognizes that in case string is Nil, the right hand side of the expression string || raise will be evaluated. Since typeof(raise "Empty input") is NoReturn the execution would not return to the current scope in that case. That leaves only String as resulting type of the expression.

Every expression whose code paths all result in NoReturn will be NoReturn as well. NoReturn does not show up in a union type because it would essentially be included in every expression's type. It is only used when an expression will never return to the current scope.

NoReturn can be explicitly set as return type of a method or function definition but will usually be inferred by the compiler. NOTE: This is a pseudo-class provided directly by the Crystal compiler. It cannot be reopened nor overridden.