Typedefs

Domains: Dart

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

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

class SortedCollection { 
   Function compare; 
   SortedCollection(int f(Object a, Object b)) { 
      compare = f; 
   } 
} 

// Initial, broken implementation. 
int sort(Object a, Object b) => 0; void main() { 
   SortedCollection coll = SortedCollection(sort); 

   // All we know is that compare is a function, but what type of function? 
   assert(coll.compare is Function); 
} 

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

typedef Compare = int Function(Object a, Object b); 

class SortedCollection { 
   Compare compare; SortedCollection(this.compare); 
} 

// Initial, broken implementation. 
int sort(Object a, Object b) => 0; 

void main() { 
   SortedCollection coll = SortedCollection(sort); 
   assert(coll.compare is Function); 
   assert(coll.compare is Compare); 
}

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 type of any function. For example:

typedef Compare<T> = int Function(T a, T b); 
int sort(int a, int b) => a - b; 

void main() { 
   assert(sort is Compare<int>); // True! 
}

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