Typedefs

In Dart, functions are objects, just like strings and numbers areobjects. A typedef, or function-type alias, gives a function type aname that you can use when declaring fields and return types. A typedefretains type information when a function type is assigned to a variable.

Consider the following code, which doesn’t use a typedef:

  1. class SortedCollection {
  2.   Function compare;
  4.   SortedCollection(int f(Object a, Object b)) {
  5.     compare = f;
  6.   }
  7. }
  9. // Initial, broken implementation.
  10. int sort(Object a, Object b) => 0;
  12. void main() {
  13.   SortedCollection coll = SortedCollection(sort);
  15.   // All we know is that compare is a function,
  16.   // but what type of function?
  17.   assert(coll.compare is Function);
  18. }

Type information is lost when assigning f to compare. The type off is (Object, Object) → int (where → means returns), yet thetype of compare is Function. If we change the code to use explicitnames and retain type information, both developers and tools can usethat information.

  1. typedef Compare = int Function(Object a, Object b);
  3. class SortedCollection {
  4.   Compare compare;
  6.   SortedCollection(this.compare);
  7. }
  9. // Initial, broken implementation.
  10. int sort(Object a, Object b) => 0;
  12. void main() {
  13.   SortedCollection coll = SortedCollection(sort);
  14.   assert(coll.compare is Function);
  15.   assert(coll.compare is Compare);
  16. }

Note: Currently, typedefs are restricted to function types. We expect this to change.

Because typedefs are simply aliases, they offer a way to check the typeof any function. For example:

  1. typedef Compare<T> = int Function(T a, T b);
  3. int sort(int a, int b) => a - b;
  5. void main() {
  6.   assert(sort is Compare<int>); // True!
  7. }