I was surprised to find the below code doesn't typecheck even with -XLiberalTypeSynonyms. Am I missing something or is this really not possible?
Reading the error message would help: * The type synonym `Foo' should have 2 arguments, but has been given 1 -XLiberalTypeSynonyms relaxes the 'saturated application' rule a little https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/liberal_type_synonym... "You can apply a type synonym to a partially applied type synonym:" But that isn't what you're doing. The docs say that still " ... malformedness ... * Partially-applied type synonym." Type Synonyms are not functions. Think of them as 'macro' expansions. Possibly your `Bar` could be a type synonym, taking `(Foo Maybe)` as an argument in the `instance Eq`. What type is it exactly that you want to derive an instance for?