So as others noted; this code will compile, but with a warning. Turn on warnings for incomplete patterns (-fwarn-incomplete-patterns), allow warnings to break c

So as others noted; this code will compile, but with a warning. Turn on warnings for incomplete patterns (-fwarn-incomplete-patterns), allow warnings to break compilation (-Werror), and you've got compile time safety. https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/options-sanity.html https://www.haskell.org/ghc/docs/7.0.4/html/users_guide/opti... lists out what the flags do.