Michael Mossey wrote:
For debugging purposes I'm interested in pretty-printing data; to start with, lists of algebraic data types. Basically I'd like 'show' with the ability to put each entry of a list on a separate line, and indented. Note that the algebraic data might have an inner list as one of its elements, so this is a non-obvious formatting problem.
I believe I can make instances of Show, can I not? Is there something called showList which I can use to code my own method of showing lists of a particular type? My understanding is that I can't make [a] an instance of Show; hence they provided showList.
For your own data type say "Foo" you can provide your own showList definition, that will be used whenever you show something of type "[Foo]". You cannot rewrite the (generic) "instance Show a => Show [a]". instance Show Foo where show _ = "Foo" showList l s = unlines (map ((" " ++) . show) l) ++ s With overlapping instances you could rewrite "instance Show [Foo]", but you should prefer the above or start with a separate class "Pretty" and use show as default implementation: class Show a = Pretty a pretty :: a -> String pretty = show With some ghc extension (undecidable instances?) you can get instances for all types: instance Show a = Pretty a Provide a list instance: instance Pretty a => Pretty [a] where pretty l = ... and you can write (overlapping) instances for other types. For your own data types use "deriving Show" and provide a Pretty instance (if you don't like the Show result). HTH Christian