export constructors only for matching (Was: qualified exports)

[Henning Thielemann]

On Fri, 22 Feb 2019, Bryan Richter wrote:

> > Another pet peeve: Export constructors but only for pattern matching purposes, i.e., elimination; not for
> construction--we'd export smart constructors for the latter which would ensure invariants
> 
> I believe you are in luck. Since 7.8.1, GHC supports "unidirectional pattern synonyms", as described somewhere in
> this section:
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#extension-PatternSynonyms

In some cases you can even solve it in Haskell 98.

If you have

    data T = Cons Char

and you want to let users match Cons without exporting it, you may export 
a "destructor" like

    uncons :: T -> Char
    uncons (Cons a) = a

or you export a continuation passing function:

    withT :: (Char -> a) -> T -> a
    withT f (Cons a) = f a

and the user would no longer write

    g :: Int -> T -> Ordering -> Bool
    g x (Cons y) z = foo x y z

but

    g :: Int -> T -> Ordering -> Bool
    g x = withT $ \y z -> foo x y z