Show and Read instance

What shall the Show and Read instances show and parse, respectively? It is certainly a good thing, if

• read parses, what show shows.

The next question is: Shall show emit valid Haskell code, that represents the shown data, or shall it show eye-candy, maybe even formatted in a line-oriented way.

• Meaning of Show and Read I
• Meaning of Show and Read II
• Show instance for Tree
• Raw vs. Show