What is ad-hoc polymorphism?

Answer 1

If the range of actual types that can be used is finite and the combinations must be specified individually prior to use, it is called ad-hoc polymorphism.

Ad-hoc polymorphism usually refers to simple overloading, but sometimes automatic type conversion, known as coercion, is also considered to be a kind of ad-hoc polymorphism. Common to these two types is the fact that the programmer has to specify exactly what types are to be usable with the polymorphic function.

The name refers to the manner in which this kind of polymorphism is typically introduced: "Oh, hey, let's make the + operator work on strings, too!" Some argue that ad-hoc polymorphism is not polymorphism in a meaningful computer science sense at all---that it is just syntactic sugar for calling append_integer, append_string, etc., manually. One way to see it is that

• to the user, there appears to be only one function, but one that takes different types of input and is thus type polymorphic; on the other hand,
• to the author, there are several functions that need to be written---one for each type of input---so there's essentially no polymorphism.

Example

Imagine, if you will, an operator + that may be used in the following ways:

1. 1 + 2 → 3
2. 3.14 + 0.0015 → 3.1415
3. 1 + 3.7 → 4.7
4. [1, 2, 3] + [4, 5, 6] → [1, 2, 3, 4, 5, 6]
5. [true, false] + [false, true] → [true, false, false, true]
6. "foot" + "ball" → "football"