Haskell: deriving Show for Fix types

Question

I'm trying to implement a recursive datatype using recursion-schemes. I would like to be able to print it.

import Data.Functor.Foldable

data T1F a = Foo deriving Show
type T1 = Fix T1F
data T2 = Bar T1 deriving Show -- error here

Error message:

No instance for (Data.Functor.Classes.Show1 T1F)
  arising from the first field of ‘Bar’ (type ‘T1’)
Possible fix:
  use a standalone 'deriving instance' declaration,
    so you can specify the instance context yourself
When deriving the instance for (Show T2)

How do I make T1 derive Show?

The error message tells you the problem. You need an instance of Show1 for T1F. Writing instances of Show1 by hand can be quite tedious. Fortunately the deriving-compat package has got your back. — Benjamin Hodgson
@BenjaminHodgson is there an example of deriving it manually? — CMCDragonkai
@CMCDragonkai Easiest way is to derive Show for your datatype, look at the generated code by compiling with -ddump-deriv, and then paste and adapt it. Not a fun task, though - much easier to use the TH helpers in the pkg! — Benjamin Hodgson

Carl Patenaude Poulin Carl Patenaude Poulin · Accepted Answer · 2017-03-21T19:50:02

Using the package deriving-compat:

{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Foldable
import Text.Show.Deriving

data T1F a = Foo deriving Show
$(deriveShow1 ''T1F)
type T1 = Fix T1F
data T2 = Bar T1 deriving Show

Haskell: deriving Show for Fix types

1 Answers