Case functions give the programmer a way to create different versions of a function; the case that is called is based on the arguments given when the function is called. As with
match statements, the function can be selected by the value or the type of the arguments, and guard statements can be used for additional control. Different cases of a function have the same name and parameter list with the same number of arguments (though not necessarily the same parameter types or return types).
A classic example of this type of function is the a recursive function with a base case. For example, the factorial of
n can be found using the following function:
/ f(0) => 1 f(n) = | \ f(n) => n * f(n - 1)
This could be implemented in Pony with an
if statement to check whether to return 1 or make the recursive call, but it can also be implemented with a set of case functions. The
fac_case function uses parameter matching to dispatch based on whether or not the argument is
0, while the
fac_guard function uses a guard statement to test the incoming argument. In both functions, there is a default case in which there are no match parameters nor guard statements.
primitive Factorial fun fac_conditional(n: U64): U64 => if n == 0 then 1 else n * fac_conditional(n - 1) end // dispatch based on argument match fun fac_case(0): U64 => 1 fun fac_case(n: U64): U64 ? => n * (fac_case(n - 1)? as U64) // dispatch based on guard function fun fac_guard(n: U64): U64 if n == 0 => 1 fun fac_guard(n: U64): U64 ? => n * (fac_guard(n - 1)? as U64) actor Main new create(env: Env) => let n: U64 = try env.args(1)?.read_int[U64]()?._1 else 13 end try env.out.print(n.string().add("! = ") .add(Factorial.fac_conditional(n).string())) env.out.print(n.string().add("! = ") .add((Factorial.fac_case(n)? as U64).string())) env.out.print(n.string().add("! = ") .add((Factorial.fac_guard(n)? as U64).string())) end
Note that in the example above the return values of
fac_guard(n) must explicitly be cast as
U64. This is because the compiler does not check to make sure that all cases can be matched and instead creates an implicit base case that returns None if no match occurs. Therefore, if
T is the union of all of the explicit return types for a case function, then the implied type of the function is actually
(T | None).
The parameter types for different cases do not have to match, but each case of the same function must have the same number of parameters, and the parameters must have the same names in each case of the function. Also, different cases of the same function can have different return types; as mentioned above, the return types are combined as a union (along with None) to create the final type for the function. The type and name of each parameter must be specified somewhere in the combination of case functions, though not necessarily all in the same case (see the
b parameters in the
_fizz_buzz method below).
In the following implementation of FizzBuzz
fizz_buzz is a case function that takes either a
U64 or a
Range[U64], and consequently returns either a
String or an
Array[String], while the helper function
_fizz_buzz is a case function that determines what to return by matching the arguments that are passed to it:
use "collections" class FizzBuzz // case functions for FizzBuzz fun _fizz_buzz(_, 0, 0): String => "FizzBuzz" fun _fizz_buzz(_, 0, b: U64): String => "Fizz" fun _fizz_buzz(_, f: U64, 0): String => "Buzz" fun _fizz_buzz(x: U64, _, _): String => x.string() // parameters are different, return types are different fun fizz_buzz(x: U64): String ? => _fizz_buzz(x, x % 3, x % 5) as String fun fizz_buzz(x: Range[U64]): Array[String] ? => let acc = Array[String] for z in x do acc.push(fizz_buzz(z)? as String) end actor Main new create(env: Env) => try for x in Range[U64](1, 101) do env.out.print(FizzBuzz.fizz_buzz(x)? as String) end let lines = FizzBuzz.fizz_buzz(Range[U64](1, 101))? as Array[String] env.out.print("\n".join(lines)) end
It is important to note that case functions resolve overlapping cases by using the first function that matches. For example, the prints
foo bar because the case of
foo matching the type
Foo is declared before the one that matches
Bar, and the case of
bar matching the type
Bar is declared before the one that matches
interface Foo class Bar is Foo new ref create() => None actor Main fun foo(x: Foo): String => "foo" fun foo(x: Bar): String => "bar" fun bar(x: Bar): String => "bar" fun bar(x: Foo): String => "foo" new create(env: Env) => let x = Bar try env.out.print((foo(x) as String) + " " + (bar(x) as String)) end
Case functions can improve readability by making it clear that certain argument will cause certain code paths to execute. They are currently implemented as sugar that translates to a
match statement, so there is no more overhead than if the programmer had written the
Does Pony support overloaded functions? There is no strict definition of "overloaded function" so it is difficult to give a definitive answer to this question. Case functions provide a way to supply several functions of the same name with behavior that varies based on the types and values of their arguments; for some people, these look like overloaded functions. However, it is important to understand the differences between case functions and some of the traditional expectations of overloaded functions.
- The current implementation of case functions does not do an exhaustive match to determine the return type of the synthesized function, so the return type is always a union that includes
Noneas one of the types.
- Case functions do not provide a way to specify functions with the same name but different argument counts.
- Overlapping matches are handled in the order in which the functions were declared. Depending on your needs, case functions may or may not offer an acceptable alternative to overloaded functions.