On 17 May 2010 12:56, Abby Henríquez Tejera
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