On 17 May 2010 12:56, Abby Henríquez Tejera wrote:

I'm a Haskell newbie and there's a bit of Haskell code that I don't understand how it works. In the prelude, defining the class Show, the function showList is implemented twice, one for String and another one for other lists:

   showList cs = showChar '"' . showl cs                 where showl ""       = showChar '"'                       showl ('"':cs) = showString "\\\"" . showl cs                       showl (c:cs)   = showLitChar c . showl cs

This is the default implementation; if an instance doesn't define showList then this value is used.

   showList []       = showString "[]"    showList (x:xs)   = showChar '[' . shows x . showl xs                        where showl []     = showChar ']'                              showl (x:xs) = showChar ',' . shows x .                                             showl xs

This is how the Show instance for Char defines showList (i.e. it overrides the default one). There would then be something like: instance (Show a) => Show [a] where show = showList So, depending on the type used, it will either use the special ".." method (for String = [Char]) or the default one (or another special one if another data type overrides the default implementation for Show). See http://book.realworldhaskell.org/read/using-typeclasses.html for more information. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

Your question is actually deeper than some of the people answering you seem to realize. How does ghci decide what to do when you say show [] ? The expression [] has type [a], which means it could be a list of any type 'a', including Char. Normally, when Haskell can't determine the type in this kind of context it will complain. You can try compiling the program main = putStrLn (show []) and you'll see the error message. But in ghci there is a special defaulting rule that will use the type () ambiguous types. So ghci will use the 'Show ()' instance, which uses the default implementation for showList. -- Lennart On Mon, May 17, 2010 at 4:56 AM, Abby Henríquez Tejera wrote:

Hi.

I'm a Haskell newbie and there's a bit of Haskell code that I don't understand how it works. In the prelude, defining the class Show, the function showList is implemented twice, one for String and another one for other lists:

   showList cs = showChar '"' . showl cs                 where showl ""       = showChar '"'                       showl ('"':cs) = showString "\\\"" . showl cs                       showl (c:cs)   = showLitChar c . showl cs

and

   showList []       = showString "[]"    showList (x:xs)   = showChar '[' . shows x . showl xs                        where showl []     = showChar ']'                              showl (x:xs) = showChar ',' . shows x .                                             showl xs

The thing is... how does Haskell «know» which to execute? It works even for the blank string: Prelude> show "" "\"\"" Prelude> show [] "[]"

Salud, Abby _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe