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

[Brandon S. Allbery KF8NH]

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 at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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