Generic versions of TextShow and TextShow1 class functions, as an alternative to TextShow.TH, which uses Template Haskell. Because there is no Generic2 class, TextShow2 cannot be implemented generically.

This implementation is based off of the Generics.Deriving.Show module from the generic-deriving library.

Since: 2

TextShow instances can be easily defined for data types that are Generic instances. The easiest way to do this is to use the DeriveGeneric extension.

{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
import TextShow
import TextShow.Generic

data D a = D a
  deriving (Generic, Generic1)

instance TextShow a => TextShow (D a) where
    showbPrec = genericShowbPrec

instance TextShow1 D where
    showbPrecWith = genericShowbPrecWith

Suppose you intend to use genericShowbPrec to define a TextShow instance.

data Oops = Oops
    -- forgot to add "deriving Generic" here!

instance TextShow Oops where
    showbPrec = genericShowbPrec

If you forget to add a deriving Generic clause to your data type, at compile-time, you will get an error message that begins roughly as follows:

No instance for (GTextShow (Rep Oops))

This error can be confusing, but don't let it intimidate you. The correct fix is simply to add the missing "deriving Generic" clause.

Similarly, if the compiler complains about not having an instance for (GTextShow1 (Rep1 Oops1)), add a "deriving Generic1" clause.