[Haskell-beginners] Just clarifying the "pred" and "succ" functions in Haskell

Andy Elvey 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 -
- Andy

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