CODEWITHSUNDEEP
functional-programmingsmlsmlnj
Mouhyi
Mouhyi

Reputation: 377

SML Value Restriction - Heap

Basically, I want a function to make a heap from a node and 2 subheaps. The heap representation is as follows (where the int in the Cons represents the rank of the node)

datatype 'a heap = Empty | Heap of int * 'a * 'a heap *  'a heap

and my function is:

fun makeHeap x h1 h2 = Heap ((rank h1)+1, x, h1, h2)

The programs comipiles but when I call makeHeap, I get a weird value instead of a Heap:

makeHeap(#"c", Empty, Empty);
stdIn:72.1-72.29 Warning: type vars not generalized because of
   value restriction are instantiated to dummy types (X1,X2,...)
val it = fn
 : (char * ?.X1 heap * ?.X2 heap) heap
    -> (char * ?.X1 heap * ?.X2 heap) heap
      -> (char * ?.X1 heap * ?.X2 heap) heap

Upvotes: 0

Views: 370

Answers (1)

Andreas Rossberg
Andreas Rossberg

Reputation: 36088

You have defined makeHeap in curried form but call it with tupled arguments. The two forms imply different types and thus are not interchangeable. Simply change the call to

makeHeap #"c" Empty Empty

Alternatively, change the definition to

fun makeHeap(x, h1, h2) = ...

Upvotes: 3

Related Questions