Only the first constructor needs to be inspected:
headZig :: Zig a b -> a
headZig (Cins x _) = x
headZig Nil = error "headZig: empty input"

headZag :: Zag a b -> a
headZag (Cans x _) = x
headZag Nal = error "headZag: empty input"
The first type parameter is the type of the first element in either algebraic data type. The source examples therefore return 1 and "C", respectively. These functions are partial at empty inputs, as allowed by the stated nonempty-input requirement.

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