Haskell: Show screwed up?

Question

The show function in Haskell doesn\'t seem to do what it should:

Prelude> let str = \"stack\\n\\noverflow\"
Prelude> putStrLn str
stack


overfl         


        

Answer 1

Porges' plan works and I think it brings out brings out what show is really up to, since the confusing behavior you found in ghci will still turn up if you get the IO function you want. Note that I added an instance for Char to Porges' code, since you would presumably want that not to have quotes.

{-# LANGUAGE UndecidableInstances, OverlappingInstances,
         FlexibleInstances, TypeSynonymInstances #-}
class ToString a where
  toString :: a -> String

instance ToString String where
  toString = id

instance ToString Char where
  toString x = [x]

instance Show a => ToString a where
  toString = show

foo :: (ToString a) => a -> IO ()
foo x = do {putStrLn $ toString x}

then, in ghci, watch what happens with foo.show:

*Main> let str =  "stack\n\noverflow"   
*Main> show str   
"\"stack\\n\\noverflow\""    
*Main> putStrLn str   
stack  

overflow  
*Main> putStrLn (show str)  
"stack\n\noverflow"  
*Main> foo str   
stack  

overflow  
*Main> foo (show str)  
"stack\n\noverflow"  
*Main> foo ( show (show str))  
"\"stack\\n\\noverflow\""  
*Main> let newl = "\n"  
*Main> foo newl  


*Main> putStrLn newl  


*Main> putStrLn (show newl)  
"\n"   



*Main> foo (show newl)  
"\n"  
*Main> foo (show (show newl))  
"\"\\n\""  
*Main>