On Feb 6, 2010, at 00:27 , Andy Elvey wrote:
However, is my understanding correct that this can be extended to lists (arrays in C) so that (for example) for a list ["foo", "bar", "baz"] , "pred "bar" " would give you "foo" , and "succ "bar" " would give you "baz"?
No. Leaving aside that you don't manipulate lists that way in Haskell, "bar" is a random value of type String (which is [Char]), not a member of an enumeration. For comparison:
data MyType = Foo | Bar | Baz deriving Enum; -- pred Bar = Foo, succ Bar = Baz
Some languages (e.g. Perl) do give an enumerable value to Strings, but `succ "Bar"' would be something like "Baq". (This could be done in Haskell, with some pain; it starts with `instance (Enum a, Bounded a) => Enum [a] where...'.) You can't go from a string like "Bar" to whatever lists might contain that string (and what if multiple lists contained it?), so there's no way to get an interpretation like that; you would need an enumerator which had access both to the list and the member in question, whereas Enum has access only to the type. (There exist dependent type systems where you could encode that information into a defined (sub)type, but Haskell doesn't support it directly.) What you *can* do is that, because the types of list and array indexes are members of Enum, you can for example use Data.List.index to determine the index (if any!) of that item in your list, then take prev or succ of that. Beware of running off the end of the list, though. (It's also more complicated for arrays because array indexes are themselves defined by a typeclass `Ix'.) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH