Null conditional in Motoko?

Question

Getting the following error:

expression of type
  ?Product/798
cannot produce expected type
  {id : Nat; name : Text; price : Nat; sku : Text}

when trying to loop over a list of ids, get the corresponding HashMap value, and add to a new array only if the get() function returns a Product.

for (id in productIds.vals()) {
   var product: ?Product = products.products.get(id);

   if (product != null) {
      products_list := Array.append<Product>(products_list, [product]);
   };
};

Am I maybe misunderstanding the best way to approach this? It seems like I shouldn't get any errors because if it's not a Product type (which is the "expected type" it references) it should just continue on, right?

Answer 1

The variable product is of type ?Product, but your instantiation of the Array.append function expects an array of values of plain type Product.

The comparison with null does not magically change the type of the variable. You'll need to pattern-match it with a switch to extract the actual value:

switch product {
  case (?p) { products_list := Array.append<Product>(products_list, [p]) };
  case null { ...do something else... };
}