The enum operator is a generic operator that takes one type parameter that represents the type of the enum to convert to. When it converts to an enumerated type, type inference attempts to determine the type of the enum that you want to convert to. In the following example, the variable col1 is not explicitly annotated, but its type is inferred from the later equality test. Therefore, the compiler can deduce that you are converting to a Color enumeration. Alternatively, you can supply a type annotation, as with col2 in the following example.
type Color =
| Red = 1
| Green = 2
| Blue = 3
// The target type of the conversion cannot be determined by type inference, so the type parameter must be explicit.
let col1 = enum<Color> 1
// The target type is supplied by a type annotation.
let col2 : Color = enum 2
You can also specify the target enumeration type explicitly as a type parameter, as in the following code:
Note that the enumeration casts work only if the underlying type of the enumeration is compatible with the type being converted. In the following code, the conversion fails to compile because of the mismatch between int32 and uint32.
// Error: types are incompatible
let col4 : Color = enum 2u
