From the department of improbable syntax/I don't get how this works, but it does:
> f :: [a] -> [a]
> ;; f xs = xs ++ reverse xs
Nicely (to my eye) groups/indents the binding under the signature. Especially handy with ScopedTypeVariables, to show how the scoped vars 'belong'
> f :: forall a. [a] -> [a]
> ;; f xs = xs ++ ys
> where
> ys :: [a]
> ys = reverse xs
The doubled `;;` stands out like a ditto. It could be a single semicolon (which means there's an empty decl/binding between the semis -- I don't see that's allowed by the syntax).
Unfortunately, for multi-line bindings you need to repeat the `;;`
> f :: [a] -> [a]
> ;; f [] = []
> ;; f xs = xs ++ reverse xs