Error SML NJ-Operator and Operand don't agree - ml

Question

When I try to compile my ML Program ,I get an error saying:"Operator and Operand don't agree".
candidates.sml:78.8-78.40 Error: operator and operand don't agree [tycon mismatch]
operator domain: int * (int * 'Z * 'Y) list
operand: int * (int * real) list
in expression:
tr (n,candidates)
I understand the error but I can't find a solution.
The part of the code where I get the error is:
fun agonas fileName =
let
fun tr(n,[])=[]
| tr(n,((a,b,c)::d))=((n-a+1),b,c)::(tr(n,d))
val (n,l,candidates) = parse fileName
val cand = tr(n,candidates)
in
my_solution(l,cand)
end;
,where the candidates are related with the part below:
fun parse file =
let
(* a function to read an integer from an input stream *)
fun next_int input =
Option.valOf (TextIO.scanStream (Int.scan StringCvt.DEC) input)
(* a function to read a real that spans till the end of line *)
fun next_real input =
Option.valOf (TextIO.inputLine input)
(* open input file and read the two integers in the first line *)
val stream = TextIO.openIn file
val n = next_int stream
val l = next_int stream
val _ = TextIO.inputLine stream
(* a function to read the pair of integer & real in subsequent lines *)
fun scanner 0 acc = acc
| scanner i acc =
let
val d = next_int stream
val (SOME v) = Real.fromString (next_real stream)
in
scanner (i - 1) ((d, v) :: acc)
end
in
(n, l, rev(scanner n []))
end;
fun my_solution ( n , l ,candidates ) = [2 ,3 ,5 ,4 ,6]
fun agonas fileName = my_solution ( parse fileName )
I would appreciate it if you could find the mistake.Thanks in advance.

Answer 1

The problem is that parse, using scanner, builds a list of pairs — (int * real) list — while tr expects to get a list of triples — (int * 'Z * 'Y) list.
Not knowing what tr is supposed to do, the quick and dirty fix would be to change
tr(n,((a,b,c)::d))=((n-a+1),b,c)::(tr(n,d))
into
tr(n,((a,b)::d))=((n-a+1),b)::(tr(n,d))
But that may be the wrong solution - it depends on what the code is supposed to do.
(Sometimes it helps to explicitly write out the types yourself - even before writing the code - instead of relying on type inference to catch the places where you need to do some more thinking.)

Answer 2

The error message says it all: the offending line calls trans, and that is a function that expects two arguments, the second being a list of triples. However, you are passing it a list of pairs instead (as produced by your scanner function).
You didn't show us the trans function, so I cannot be more specific about what the appropriate fix would be.