What is wrong with Show?

Is it for:

• Serialization?
• Display, eg in a user interface?
• Debugging / use in the repl?

Serialization

• Show-serialized values will not be understood by non-Haskell programs
• Inefficient
• (Real World Haskell argues that Show is good for serialization!)
• Better served by other serialization libraries

Display

• Doesn't really make sense as an abstraction. Eg Parsec's error messages: "unexpected ')', expected new line or end of input"
• Too many sensible implementations for some types, eg dates/times
• Too easy to show the wrong thing
• Impossible to change in a non-breaking way
• Better served by individual functions

Debugging / the repl

• Lots of types don't have instances: (->), IORef, IO
• Poor display for large / complex values
• No configurability in how printing is done
• Better served by ...?

A replacement for Show

class Debug a where
  debug :: a -> Repr

To recap, it should:

• explicitly only be for use in the repl and debugging
• be possible to give every type of kind Type an instance
• use a type with more structure than String
• allow us to perform configurable pretty-printing in the repl