ML. "Unzipping" list of tuples.

Question

So I have this list of tuples(n=2), witch I'm supposed to "unzip" and by that create a new list like so: for list of tuples like this val it = (1,"one") :: (2,"two") :: nil : (int,string) alterlist, the unzip function will create a list like so [(1,2),("one", "two")]. This is what I got so far:

datatype ('a, 'b) alterlist = nil | :: of ('a*'b) * ('a, 'b) alterlist; 
infixr 5 :: 

fun build4(x, one, y, two) = (x,one)::((y,two)::nil);

fun unzip(alterlist) = 
let 
    fun extract([], i) = []
        | extract((x,y)::xs, i) = if i=0 then x::extract(xs, i)
            else y::extract(xs, i);
in
    (extract(alterlist, 0))::(extract(alterlist, 1))
end;

But I get a bunch of errors:

stdIn:48.6-50.26 Error: parameter or result constraints of clauses don't agree [tycon mismatch]
  this clause:      ('Z,'Y) alterlist * 'X -> 'W
  previous clauses:      'V list * 'U -> 'W
  in declaration:
    extract =
      (fn (nil,i) => nil
        | (<pat> :: <pat>,i) =>
            if <exp> = <exp> then <exp> :: <exp> else <exp> :: <exp>)
stdIn:49.41-49.58 Error: operator and operand don't agree [tycon mismatch]
  operator domain: 'Z list * 'Y
  operand:         ('X,'W) alterlist * int
  in expression:
    extract (xs,i)
stdIn:50.9-50.26 Error: operator and operand don't agree [tycon mismatch]
  operator domain: 'Z list * 'Y
  operand:         (_ * _,'X) alterlist * int
  in expression:
    extract (xs,i)
stdIn:48.6-50.26 Error: right-hand-side of clause doesn't agree with function result type [tycon mismatch]
  expression:  (_,_) alterlist
  result type:  'Z list
  in declaration:
    extract =
      (fn (nil,i) => nil
        | (<pat> :: <pat>,i) =>
            if <exp> = <exp> then <exp> :: <exp> else <exp> :: <exp>)

And being that I'm new to ml I have very little idea what causes it. Would greatly appreciate help!

score 2 · Accepted Answer · answered May 23 '14 at 21:43

My advice is first do it without the infix operator because it is a little bit confusing. First solve it without and add it later.

Here is a solution without the infix:

  datatype ('a,'b)alterlist =   Nil 
                            |   element of 'a*('b,'a)alterlist;                         

fun unzip (Nil : ('a,'b)alterlist ) = ([],[])
  | unzip (ablist : ('a,'b)alterlist)  =
    let 
    fun extract  Nil = []
      | extract (element (curr, Nil)) = curr::[]
      | extract (element (curr, element(_,rest))) = curr::(extract rest)
    val element (_, balist) = ablist
    in
    (extract ablist, extract balist)
    end;

So for example a list like this: ["a", 1, "b", 2] would be created by:

element ("a", element (1, element ("b", element (2, Nil))));

Giving you:

val it = element ("a",element (1,element #)) : (string,int) alterlist

* The # is just to indicate the list is longer than is displayed.

And then if you would try unzip it; You would get:

val it = (["a","b"],[1,2]) : string list * int list

As you would like.

Now try to change element into a :: infix operator.

Good luck!

Your'e welcome @BobSacamano , But if the answer helped you, mark it as an answer (checkmark). — akrabi, May 24 '14 at 16:15
@BobSacamano , There's a checkmark near the answer. click it and then click the upwards arrow near it too. — akrabi, May 24 '14 at 16:23
I checkmarked it but I don't have enough street cred to vote your answer up. — Bob Sacamano, May 24 '14 at 16:28

ML. "Unzipping" list of tuples.

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