On 17 May 2010 12:56, Abby Henríquez Tejera <parad...@gmail.com> 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.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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