How to show strings in custom Prelude library Protolude

I read this blog post on how to create a custom Prelude library. The library can be found here. One of the things it does is prohibit String. It also defines a function for automatic string conversions (here). I have enabled OverloadedStrings in the cabal file.

Before using this library I had:

data Point = Point Int Int
instance Show Point where
  show (Point x y) = "(" ++ show x ++ ", " ++ show y ++ ")"

After using the library it says: "show' is not a (visible) method of classShow'"

So I resorted to create a custom function to show the data type:

showPoint :: Point -> LText
showPoint (Point x y) = toS ("(" ++ show x ++ ", " ++ show y ++ ")")

The compiler is saying that the use of toS, "(", show is ambiguous, but I don't understand why. Do I have to do something like what is proposed here?

Edit:

Had to disable OverloadedStrings and change the code to the following:

showPoint :: Point -> LText
showPoint (Point x y) = toS "(" <> show x <> toS ", " <> show y <> toS ")"

Wondering if it is possible to do the same without disabling OverloadedStrings so I don't have to use toS for every String.

Solution

This is what worked for me:

{-# LANGUAGE OverloadedStrings #-}

module Test where

import Protolude
import qualified Base as PBase

data P = P Int

instance PBase.Show P where
  show (P x) = "a P " ++ show x

Update

The protolude implemention of show is as a normal function (see the end of Protolude.hs):

show :: (Show a, StringConv String b) => a -> b
show x = toS (PBase.show x)

So you need a PBase.Show instance in order use protolude's show function.

Also protolude's show can return any of the string types, so you're not forcing others to use of String by defining a PBase.show instance.

Update #2

You can import the typeclass show function from GHC.Show:

{-# LANGUAGE NoImplicitPrelude #-}

import Protolude
import GHC.Show

data P  = P Int

instance Show P where
  show (P x) = "<-- P " ++ GHC.Show.show x ++ " -->"

main = print (P 123)