mapping lists in ML

Question

I have a question about mapping lists in ML the problem seems to repeat itself, I have the current datatypes defined :

datatype 'a seq = Nil | Cons of 'a * (unit -> 'a seq);
datatype 'a generic_list = List of 'a list
                          |Seq of 'a seq;

i'm now trying to write the following function that's supposed to recieve a "'a generic_list" and return a "int generic_list:

val rec generic_map = fn (f,List(lst)) => if lst=nil then List([])                                             
      else List(f(List.hd(lst))::generic_map(f,List( List.drop(lst,1))));

That code doesn't compile with the error of : right-hand-side of clause doesn't agree with function result type [tycon mismatch] expression:

'Z generic_list
 result type:  'Z list
 in declaration:
   generic_map =
     (fn (f,List lst) =>
           if lst = nil
           then List nil
           else List
                  (f (List.hd lst) ::
                     generic_map (f,List (List.drop (lst,1)))))

I would like to know whats the problem here and how I can fix it so it will compile, I cant find the mistake

the question is why it doesn't compile of course, i cant understand where the mistake is — yair

Andreas Rossberg Andreas Rossberg · Accepted Answer · 2013-06-14T15:12:21

In the 'else' part, you do something :: generic_map (...), which implies that generic_map would have to return a list instead of a generic_list.

Also, I don't see where you handle the seq case at all.

As a general note, I strongly suggest using pattern matching instead of if, List.hd and friends. In particular a comparison like lst = nil is always wrong, since it restricts the list to elements with equality type -- use pattern matching, or at least the List.null predicate.

mapping lists in ML

1 Answers