2024-06-24

_posts/2024-06-24-simplecov-complains-that-1-branch-is-not-covered-in-pattern-matching.md

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+---
+layout: page_with_comment
+title: "SimpleCov complains that 1 branch is not covered in pattern matching."
+date: "2024-06-24"
+tags:
+  - "ruby"
+  - "rspec"
+  - "unit"
+  - "test"
+  - "coverage"
+  - "pattern"
+  - "matching"
+---
+
+Pattern matching is a feature available in multiple programming languages that allows developers to test an expression to determine if it has certain characteristics.
+
+Ruby supports pattern matching and you can do deep matching of structured values such as arrays or hashes.
+
+Code Block 1
+```
+v = [1, 2, 3, 4]
+case v 
+in [head,*tail]
+  puts("head=#{head}, tail=#{tail}")
+else
+  puts("Not matched")
+end
+```
+
+will output "head=1, tail=[2, 3, 4]"
+
+Code Block 2
+```
+v = [1, 2, 3, 4]
+case v 
+in [head,*tail]
+  puts("head=#{head}, tail=#{tail}")
+in []
+  puts("Not matched")
+end
+```
+
+will also output "head=1, tail=[2, 3, 4]"
+
+Code Block 1 and 2 are equivalent but if you use SimpleCov to generate code coverage, even if you have 100% line coverage, you will notice that SimpleCov still says that you do not have 100% branch coverage and 1 branch is not covered.
+
+Why? Aren't these two blocks equivalent?
+
+No, the above two blocks are not equivalent. The missed branch in Code Block 2 is the "else" branch in ruby pattern matching. We can only view the two blocks equiavlent if `v` is array type. However, ruby is a dynamic type langauge and `v` can be nil, hashes or other types.
+
+Even if you are 100% sure that `v` must be an array, it is always good practice to have else branch in a pattern matching.