[Haskell-beginners] Just clarifying the "pred" and "succ"
functions in Haskell
andy.elvey at paradise.net.nz
Sat Feb 6 01:31:49 EST 2010
Brandon S. Allbery KF8NH wrote:
> 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'.)
Hi Brandon - thanks very much for that!
Ok. Yes, that is exactly what I was after. I was indeed thinking that
"pred" and "succ" related to the "previous" and "next" elements in a
list, but I now see that that is not the case.
So, I may look at doing what I would call "lpred" and lsucc" - the
predecessor and successor of a list element.
I'm somewhat surprised that (from what I can tell) Haskell doesn't seem
to have those two functions for a list. I may be wrong....
Anyway - thanks again. Bye for now -
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