Error code E0004
This error indicates that the compiler cannot guarantee a matching pattern for one or more possible inputs to a match expression. Guaranteed matches are required in order to assign values to match expressions, or alternatively, determine the flow of execution.
Erroneous code example:
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If you encounter this error you must alter your patterns so that every possible
value of the input type is matched. For types with a small number of variants
(like enums) you should probably cover all cases explicitly. Alternatively, the
underscore _
wildcard pattern can be added after all other patterns to match
"anything else". Example: