Can I make haskell GADT data constructor infix in derived Show?

Question

Consider two data declarations:

{-# LANGUAGE GADTs #-}

data X = Int `Y` Int deriving Show

data Z where
        W :: Int -> Int -> Z  deriving Show

main = do
         print (1 `Y` 2)
         print (3 `W` 4)

Running the above program produces:

1 `Y` 2
W 3 4

so the derived show knows that Y is infix and prints it accordingly. The :: syntax doesn't seem to allow infixness.

Is there any way to make the compiler derive show for W as infix, (other than explicitly providing a show instance for Z)? The desired output is

1 `Y` 2
3 `W` 4

I never knew you could tell the Show deriver to print a non-symbolic constructor infix. — rampion

rampion rampion · Accepted Answer · 2017-06-23T20:45:50

Not currently. GADT constructors are only flagged as infix under a specific set of conditions:

Note [Infix GADT constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We do not currently have syntax to declare an infix constructor in GADT syntax,
but it makes a (small) difference to the Show instance.  So as a slightly
ad-hoc solution, we regard a GADT data constructor as infix if
  a) it is an operator symbol
  b) it has two arguments
  c) there is a fixity declaration for it
For example:
   infix 6 (:--:)
   data T a where
     (:--:) :: t1 -> t2 -> T Int

So for a non-symbolic constructor like W, it looks like you're out of luck, but if you're willing to make it symbolic, you can just add a fixity declaration.

(hat tip to this template haskell bug thread)

Can I make haskell GADT data constructor infix in derived Show?

1 Answers