Patterns in let statement

In a let statement, the left side of = can be a pattern to match against the value on the right side. If the pattern cannot be matched, the program will abort.

You may notice a partial match warning error. This occurs because the array might contain a different number of elements than expected, but the let statement only handles the case where the array contains exactly three elements. This is referred to as a partial match. Partial matches make the program more fragile: the pattern matching may fail in other cases, leading to the program aborting. By default, partial match warnings are treated as errors to help prevent potential runtime issues.

Practically, the match expression is used more frequently than the let statement.