gistfile1.txt

module Expr

data Expr = X
          | Plus (Expr, Expr)

Show Expr where
  show X = "x"
  show (Plus (e_left, e_right)) = (show e_left) ++ " + " ++ (show e_right)

{-
$ idris
Idris> :l Expr.idr
*Expr> show (Plus (X,X))
prim__concat (Expr.Expr implementation of Prelude.Show.Show, method show X)
             (prim__concat " + " (Expr.Expr implementation of Prelude.Show.Show, method show X)) : String
-}