Pattern synonyms are really expressive but the complex version is kinda headache inducing: http://hackage.haskell.org/package/lens-4.17/docs/src/Control.Lens.C

Pattern synonyms are really expressive but the complex version is kinda headache inducing: http://hackage.haskell.org/package/lens-4.17/docs/src/Control.Lens.Cons.html#%3A< http://hackage.haskell.org/package/lens-4.17/docs/src/Contro... This creates a bidirectional pattern synonym - you can both match and construct values with it: foo :: Cons' s Char => s -> s foo (a :< b) = 'c' :< b foo other = other Pattern synonyms make completeness checking undecidable so you need to give manual hints to get a nice api. It also wreaks cost models - is foo O(1) or O(n)?