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
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