I can't think of a language that lets you do this; that is, allow you to input a list of any type as text. Some languages effectively encode the types in the parsing, for example in LISP, you know that 'foo is a symbol. It has a very limited set of data types and new types are described entirely in terms of those simple types, which makes parsing simple. But lets say you have
data Color = Red | Green | Blue deriving (Read,Show,Eq,Ord)
Now you suddenly expect "readLn" to detect the word "Green" and
interpret it differently from "1.0", restricting the type at runtime?
Do you realize how difficult this is?
What if Green is also used in a type in another module?
You need to specify the type to read, or provide a parser that works
for every type you care about.
-- ryan
On Fri, Oct 17, 2008 at 2:21 PM, leledumbo
So, what's the solution? This one:
(l::[Ord]) <- readLn
doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. -- View this message in context: http://www.nabble.com/List-as-input-tp19987726p20033244.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe