Final answer:
In F#, an F# selector (likely referring to a match expression) must have an else clause if not all possible cases are covered by the defined patterns. It's necessary for ensuring the function can return a result for every possible input value, which is crucial when dealing with discriminated unions or other scenarios where the input might not match any of the provided cases.
Step-by-step explanation:
The question refers to an F# selector, which is likely a confusion with F# pattern matching or the match expression. In F#, a match expression must have an else clause if not all possible cases for the input have been covered. For instance, when matching on a discriminated union, if you have not provided a match case for every possible union case, an else (or a wildcard pattern _) clause is required to ensure that the function is total, meaning that it can return a result for every possible input value.
Example of F# Pattern Matching Requirement
If you have a discriminated union representing traffic lights with Red, Yellow, and Green cases, your match expression must either include all three cases or provide an else clause to handle any cases not explicitly mentioned:
type TrafficLight = | Red | Yellow | Green
let action = function
| Red -> "Stop"
| Yellow -> "Prepare to stop"
| Green -> "Go"
| _ -> "Unknown color"
In the example above, the _ represents the else clause, which will match any value not covered by the preceding cases.