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 <paradoja@gmail.com> 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