I've got a generic function that needs to create a Tuple to call a function whose arguments I don't know the types of.
Something like this (except array in this example is created by some external code, so I can't just apply the function directly):
Result apply<Result, Where>(
Anything[] array,
Callable<Result, Where> fun)
given Where satisfies Anything[] => nothing;
Is there a type-safe way to implement this method and get the function to be called with the given arguments?
This cannot be done completely type-safely... but assuming that the array indeed contains elements of the correct types as they should appear in a Tuple of type Where, the following function will do the trick:
Tuple<Anything, Anything, Anything> typedTuple({Anything+} array) {
if (exists second = array.rest.first) {
return Tuple(array.first, typedTuple({ second }.chain(array.rest.rest)));
}
else {
return Tuple(array.first, []);
}
}
And apply gets implemented as:
Result apply<Result, Where>(
[Anything+] array,
Callable<Result, Where> fun)
given Where satisfies Anything[] {
value tuple = typedTuple(array);
assert(is Where tuple);
return fun(*tuple);
}
There's nothing relating the type of array to the parameters of fun, so that signature can't possibly be implemented in a type-safe way. You're not constraining the type of array at all; it could contain anything. How in principle would a type-safe implementation handle the case where fun expects [String, Integer] but array is [Boolean+]?
Related
I have discovered that Zig function parameters are constant. That means my naive function for freeing a HashMap doesn't work. You can see an example of the code here. I am wondering if the most correct Zig way is to pass dict as a function or if there is some other way in which I can make a parameter mutable.
const Dict = std.StringHashMap;
fn releaseDict(allocator: Allocator, dict: Dict(i16)) void {
var iter = dict.iterator();
while (iter.next()) |entry|
allocator.free(entry.key_ptr.*);
dict.deinit();
}
You don't. Function parameters are immutable by design:
Structs, unions, and arrays can sometimes be more efficiently passed as a reference, since a copy could be arbitrarily expensive depending on the size. When these types are passed as parameters, Zig may choose to copy and pass by value, or pass by reference, whichever way Zig decides will be faster. This is made possible, in part, by the fact that parameters are immutable.
Modifying function parameters can easily lead to unexpected results. If the parameter is passed by value (a copy of it is made), modifying it would not modify the original value.
What you want to do here is: pass a pointer to your hash map. E.g.
fn releaseDict(allocator: Allocator, dict: *std.StringHashMap(i16)) void {
// ...
}
In the following code, I'm trying to create a "function pointer" and an array of functions by regarding function names as usual variables:
proc myfunc1() { return 100; }
proc myfunc2() { return 200; }
// a function variable?
var myfunc = myfunc1;
writeln( myfunc() );
myfunc = myfunc2;
writeln( myfunc() );
// an array of functions?
var myfuncs: [1..2] myfunc1.type;
writeln( myfuncs.type: string );
myfuncs[ 1 ] = myfunc1;
myfuncs[ 2 ] = myfunc2;
for fun in myfuncs do
writeln( fun() );
which seems to be working as expected (with Chapel v1.16)
100
200
[domain(1,int(64),false)] chpl__fcf_type_void_int64_t
100
200
So I'm wondering whether the above usage of function variables is legitimate? For creating an array of functions, is it usual to define a concrete function with desired signature first and then refer to its type (with .type) as in the above example?
Also, is it no problem to treat such variables as "usual" variables, e.g., pass them to other functions as arguments or include them as a field of class/record? (Please ignore these latter questions if they are too broad...) I would appreciate any advice if there are potential pitfalls (if any).
This code is using first class function support, which is prototype/draft in the Chapel language design. You can read more about the prototype support in the First-class Functions in Chapel technote.
While many uses of first-class functions work in 1.16 and later versions, you can expect that the language design in this area will be revisited. In particular there isn't currently a reasonable answer to the question of whether or not variables can be captured (and right now attempting to do so probably results in a confusing error). I don't know in which future release this will change, though.
Regarding the myfunc1.type part, the section in the technote I referred to called "Specifying the type of a first-class function" presents an alternative strategy. However I don't see any problem with using myfunc1.type in this case.
Lastly, note that the lambda support in the current compiler actually operates by creating a class with a this method. So you can do the same - create a "function object" (to borrow a C++ term) - that has the same effect. A "function object" could be a record or a class. If it's a class, you might use inheritance to be able to create an array of objects that can respond to the same method depending on their dynamic type. This strategy might allow you to work around current issues with first class functions. Even if first-class-function support is completed, the "function object" approach allow you to be more explicit about captured variables. In particular, you might store them as fields in the class and set them in the class initializer. Here is an example creating and using an array of different types of function objects:
class BaseHandler {
// consider these as "pure virtual" functions
proc name():string { halt("base name called"); }
proc this(arg:int) { halt("base greet called"); }
}
class HelloHandler : BaseHandler {
proc name():string { return "hello"; }
proc this(arg:int) { writeln("Hello ", arg); }
}
class CiaoHandler : BaseHandler {
proc name():string { return "ciao"; }
proc this(arg:int) { writeln("Ciao ", arg); }
}
proc test() {
// create an array of handlers
var handlers:[1..0] BaseHandler;
handlers.push_back(new HelloHandler());
handlers.push_back(new CiaoHandler());
for h in handlers {
h(1); // calls 'this' method in instance
}
}
test();
Yes, in your example you are using Chapel's initial support for first-class functions. To your second question, you could alternatively use a function type helper for the declaration of the function array:
var myfuncs: [1..2] func(int);
These first-class function objects can be passed as arguments into functions – this is how Futures.async() works – or stored as fields in a record (Try It Online! example). Chapel's first-class function capabilities also include lambda functions.
To be clear, the "initial" aspect of this support comes with the caveat (from the documentation):
This mechanism should be considered a stopgap technology until we have developed and implemented a more robust story, which is why it's being described in this README rather than the language specification.
I was just wondered which is better way for returning some result of function or instance of Class?
1)
FN() {
var result = callSomeFN();
return result;
}
InitClass(int typeId): MyClass {
MyClass class = new MyClass(typeId);
return class;
}
2)
FN() {
return callSomeFN();
}
InitClass(int typeId): MyClass {
return new MyClass(typeId);
}
The only difference I see here is the declaration of a variable. I wouldn't say one is better than the other. If the variable is never going to be used why bother defining it? Just return.
I suppose if you really wanted to nitpick it would depend on if you're returning a value or reference type. If it's a reference it doesn't matter since you're going to store something in memory and return a pointer (both the variable and the return value are just pointers to the same memory space). If it's a value type it will end up copying the value and disposing of the version within the function scope. If the variable is never used why create something just to destroy it? The default behavior would be highly language dependent, which you didn't specify.
I have 2 general questions about function programming.
Consider the following 3 functions:
result = fun3(fun2(fun1(param1))); // param4
function fun1(param1) {
// ...
return param2;
}
function fun2(param2) {
// ...
return param3;
}
function fun3(param3) {
// ...
return param4;
}
Each function expects 1 parameter, does some computation and returns a variable.
In my case, every subsequent function relies on the output of the preceding function.
Is the solution in my example a common practice? Or are there better ways?
What if a function produces 2 outputs and 2 different functions need them?
Like in this example:
function fun1(param1) {
// ...
return param2, param3;
}
function fun2(param2) {
// ...
return param4;
}
function fun3(param3) {
// ...
return param5;
}
PS: Although this is a general programming questions, maybe it could be important to mention, that I use PHP.
The answer to the first question is that, yes, it's a very common technique in functional programming. It is, for example, the basis of currying which is also very common.
As regards the second question, many functional languages support the concept of a tuple or heterogeneous list - if you want to return multiple values you either explicitly create one or the runtime generates one for you on the fly. Sending return values into multiple subsequent functions would be done with the aid of a helper function which does the multiplexing. The question becomes how you handle the return values from those functions, presumably as a list I guess. It might depend on your use case.
In Python that might look like
def multiplex(functions, value):
return [func(value) for func in functions]
To keep with your single-argument style the above might end up being curried in some fashion so that the list of functions to multiplex to are bound in a separate function application. Different languages achieve that in different ways, Python has a functools module which supports partial application of a function whereas Scala natively supports partially applying a function.
I am reading about boost::function and I am a bit confused about its use and its relation to other C++ constructs or terms I have found in the documentation, e.g. here.
In the context of C++ (C++11), what is the difference between an instance of boost::function, a function object, a functor, and a lambda expression? When should one use which construct? For example, when should I wrap a function object in a boost::function instead of using the object directly?
Are all the above C++ constructs different ways to implement what in functional languages is called a closure (a function, possibly containing captured variables, that can be passed around as a value and invoked by other functions)?
A function object and a functor are the same thing; an object that implements the function call operator operator(). A lambda expression produces a function object. Objects with the type of some specialization of boost::function/std::function are also function objects.
Lambda are special in that lambda expressions have an anonymous and unique type, and are a convenient way to create a functor inline.
boost::function/std::function is special in that it turns any callable entity into a functor with a type that depends only on the signature of the callable entity. For example, lambda expressions each have a unique type, so it's difficult to pass them around non-generic code. If you create an std::function from a lambda then you can easily pass around the wrapped lambda.
Both boost::function and the standard version std::function are wrappers provided by the library. They're potentially expensive and pretty heavy, and you should only use them if you actually need a collection of heterogeneous, callable entities. As long as you only need one callable entity at a time, you are much better off using auto or templates.
Here's an example:
std::vector<std::function<int(int, int)>> v;
v.push_back(some_free_function); // free function
v.push_back(&Foo::mem_fun, &x, _1, _2); // member function bound to an object
v.push_back([&](int a, int b) -> int { return a + m[b]; }); // closure
int res = 0;
for (auto & f : v) { res += f(1, 2); }
Here's a counter-example:
template <typename F>
int apply(F && f)
{
return std::forward<F>(f)(1, 2);
}
In this case, it would have been entirely gratuitous to declare apply like this:
int apply(std::function<int(int,int)>) // wasteful
The conversion is unnecessary, and the templated version can match the actual (often unknowable) type, for example of the bind expression or the lambda expression.
Function Objects and Functors are often described in terms of a
concept. That means they describe a set of requirements of a type. A
lot of things in respect to Functors changed in C++11 and the new
concept is called Callable. An object o of callable type is an
object where (essentially) the expression o(ARGS) is true. Examples
for Callable are
int f() {return 23;}
struct FO {
int operator()() const {return 23;}
};
Often some requirements on the return type of the Callable are added
too. You use a Callable like this:
template<typename Callable>
int call(Callable c) {
return c();
}
call(&f);
call(FO());
Constructs like above require you to know the exact type at
compile-time. This is not always possible and this is where
std::function comes in.
std::function is such a Callable, but it allows you to erase the
actual type you are calling (e.g. your function accepting a callable
is not a template anymore). Still calling a function requires you to
know its arguments and return type, thus those have to be specified as
template arguments to std::function.
You would use it like this:
int call(std::function<int()> c) {
return c();
}
call(&f);
call(FO());
You need to remember that using std::function can have an impact on
performance and you should only use it, when you are sure you need
it. In almost all other cases a template solves your problem.