Is it possible to write a template or similar that can be used to automatically curry functions in D? Manually writing out all the nested delegates is killing me.
Basically, for a function f with e.g. 3 arguments, which can usually be called like f(a,b,c), I want it to be callable as f(a)(b)(c).
I know about std.functional.partial, but that's not what I want. I want to translate the function definition side, not the calling side.
I also know this is far from best practice, but I'm generating code, so bear with me.
Well, something along these lines should do the job:
template autocurry(alias what) {
import std.traits;
static if(Parameters!what.length)
auto autocurry(Parameters!what[0] arg) {
alias Remainder = Parameters!what[1 .. $];
auto dg = delegate(Remainder args) {
return what(arg, args);
};
static if(Remainder.length > 1)
return &autocurry!dg;
else
return dg;
}
else
alias autocurry = what;
}
int foo(int a, string b, float c) {
import std.stdio; writeln(a, " ", b, " ", c);
return 42;
}
string test() {
import std.stdio; writeln("called test");
return "no args";
}
void main() {
import std.stdio;
alias lol = autocurry!foo;
writeln(lol(30)("lol")(5.3));
auto partial = lol(20);
partial("wtf")(10.5);
alias t = autocurry!test;
writeln(t());
}
The idea there is pretty simple: generate the helper function based on the remaining arguments - if there are any, return the address of the helper as the delegate, otherwise, just return the delegate that calls the collected arguments. A little recursiveness handles 1+ arg cases, and the static if on the outside handles the 0 arg case by just returning the original function.
Language features to note:
eponymous templates. When a template has a member with the same name as the template (in this case, autocurry), it is automatically referenced when used.
tuple expansion. When I call what(arg, args), the args one, being a built-in tuple, is automatically expanded to create the complete argument list.
the various auto returns here (the explicit auto autocurry and the implicit delegate keyword without specifying a return type) just forward whatever other random type the body happens to return.
In the main function, I did alias lol = autocurry!foo; (I use lol as my placeholder name a lot, lol). You could also overload it at top level:
int foo(int a, string b, float c) {
import std.stdio; writeln(a, " ", b, " ", c);
return 42;
}
alias foo = autocurry!foo; // overloads the auto-curried foo with the original foo
And now you can use it directly, along side the original:
void main() {
foo(30)("lol")(5.3); // overload resolves to curried version
foo(40, "cool", 103.4); // overload resolves to original automatically
}
If you prefer a new name or the overload is up to you, either can work.
Note that each argument is liable to allocate some memory to store it for the next delegate. The GC will be responsible for cleaning that up.
Related
The Goal:
decide during runtime which templated function to use and then use it later without needing the type information.
A Partial Solution:
for functions where the parameter itself is not templated we can do:
int (*func_ptr)(void*) = &my_templated_func<type_a,type_b>;
this line of code can be modified for use in an if statement with different types for type_a and type_b thus giving us a templated function whose types are determined during runtime:
int (*func_ptr)(void*) = NULL;
if (/* case 1*/)
func_ptr = &my_templated_func<int, float>;
else
func_ptr = &my_templated_func<float, float>;
The Remaining Problem:
How do I do this when the parameter is a templated pointer?
for example, this is something along the lines of what I would like to do:
int (*func_ptr)(templated_struct<type_a,type_b>*); // This won't work cause I don't know type_a or type_b yet
if (/* case 1 */) {
func_ptr = &my_templated_func<int,float>;
arg = calloc(sizeof(templated_struct<int,float>, 1);
}
else {
func_ptr = &my_templated_func<float,float>;
arg = calloc(sizeof(templated_struct<float,float>, 1);
}
func_ptr(arg);
except I would like type_a, and type_b to be determined during runtime. I see to parts to the problem.
What is the function pointers type?
How do I call this function?
I think I have the answer for (2): simply cast the parameter to void* and the template function should do an implicit cast using the function definition (lease correct me if this won't work as I think it will).
(1) is where I am getting stuck since the function pointer must include the parameter types. This is different from the partial solution because for the function pointer definition we were able to "ignore" the template aspect of the function since all we really need is the address of the function.
Alternatively there might be a much better way to accomplish my goal and if so I am all ears.
Thanks to the answer by #Jeffrey I was able to come up with this short example of what I am trying to accomplish:
template <typename A, typename B>
struct args_st {
A argA;
B argB;
}
template<typename A, typename B>
void f(struct args_st<A,B> *args) {}
template<typename A, typename B>
void g(struct args_st<A,B> *args) {}
int someFunction() {
void *args;
// someType needs to know that an args_st struct is going to be passed
// in but doesn't need to know the type of A or B those are compiled
// into the function and with this code, A and B are guaranteed to match
// between the function and argument.
someType func_ptr;
if (/* some runtime condition */) {
args = calloc(sizeof(struct args_st<int,float>), 1);
f((struct args_st<int,float> *) args); // this works
func_ptr = &g<int,float>; // func_ptr should know that it takes an argument of struct args_st<int,float>
}
else {
args = calloc(sizeof(struct args_st<float,float>), 1);
f((struct args_st<float,float> *) args); // this also works
func_ptr = &g<float,float>; // func_ptr should know that it takes an argument of struct args_st<float,float>
}
/* other code that does stuff with args */
// note that I could do another if statement here to decide which
// version of g to use (like I did for f) I am just trying to figure out
// a way to avoid that because the if statement could have a lot of
// different cases similarly I would like to be able to just write one
// line of code that calls f because that could eliminate many lines of
// (sort of) duplicate code
func_ptr(args);
return 0; // Arbitrary value
}
Can't you use a std::function, and use lambdas to capture everything you need? It doesn't appear that your functions take parameters, so this would work.
ie
std::function<void()> callIt;
if(/*case 1*/)
{
callIt = [](){ myTemplatedFunction<int, int>(); }
}
else
{
callIt = []() {myTemplatedFunction<float, float>(); }
}
callIt();
If I understand correctly, What you want to do boils down to:
template<typename T>
void f(T)
{
}
int somewhere()
{
someType func_ptr;
int arg = 0;
if (/* something known at runtime */)
{
func_ptr = &f<float>;
}
else
{
func_ptr = &f<int>;
}
func_ptr(arg);
}
You cannot do that in C++. C++ is statically typed, the template types are all resolved at compile time. If a construct allowed you to do this, the compiler could not know which templates must be instanciated with which types.
The alternatives are:
inheritance for runtime polymorphism
C-style void* everywhere if you want to deal yourself with the underlying types
Edit:
Reading the edited question:
func_ptr should know that it takes an argument of struct args_st<float,float>
func_ptr should know that it takes an argument of struct args_st<int,float>
Those are incompatible. The way this is done in C++ is by typing func_ptr accordingly to the types it takes. It cannot be both/all/any.
If there existed a type for func_ptr so that it could take arguments of arbitrary types, then you could pass it around between functions and compilation units and your language would suddenly not be statically typed. You'd end up with Python ;-p
Maybe you want something like this:
#include <iostream>
template <typename T>
void foo(const T& t) {
std::cout << "foo";
}
template <typename T>
void bar(const T& t) {
std::cout << "bar";
}
template <typename T>
using f_ptr = void (*)(const T&);
int main() {
f_ptr<int> a = &bar<int>;
f_ptr<double> b = &foo<double>;
a(1);
b(4.2);
}
Functions taking different parameters are of different type, hence you cannot have a f_ptr<int> point to bar<double>. Otherwise, functions you get from instantiating a function template can be stored in function pointers just like other functions, eg you can have a f_ptr<int> holding either &foo<int> or &bar<int>.
Disclaimer: I have already provided an answer that directly addresses the question. In this answer, I would like to side-step the question and render it moot.
As a rule of thumb, the following code structure is an inferior design in most procedural languages (not just C++).
if ( conditionA ) {
// Do task 1A
}
else {
// Do task 1B
}
// Do common tasks
if ( conditionA ) {
// Do task 2A
}
else {
// Do task 2B
}
You seem to have recognized the drawbacks in this design, as you are trying to eliminate the need for a second if-else in someFunction(). However, your solution is not as clean as it could be.
It is usually better (for code readability and maintainability) to move the common tasks to a separate function, rather than trying to do everything in one function. This gives a code structure more like the following, where the common tasks have been moved to the function foo().
if ( conditionA ) {
// Do task 1A
foo( /* arguments might be needed */ );
// Do task 2A
}
else {
// Do task 1B
foo( /* arguments might be needed */ );
// Do task 2B
}
As a demonstration of the utility of this rule of thumb, let's apply it to someFunction(). ... and eliminate the need for dynamic memory allocation ... and a bit of cleanup ... unfortunately, addressing that nasty void* is out-of-scope ... I'll leave it up to the reader to evaluate the end result. The one feature I will point out is that there is no longer a reason to consider storing a "generic templated function pointer", rendering the asked question moot.
// Ideally, the parameter's type would not be `void*`.
// I leave that for a future refinement.
void foo(void * args) {
/* other code that does stuff with args */
}
int someFunction(bool condition) {
if (/* some runtime condition */) {
args_st<int,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
else {
args_st<float,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
return 0;
}
Your choice of manual memory management and over-use of the keyword struct suggests you come from a C background and have not yet really converted to C++ programming. As a result, there are many areas for improvement, and you might find that your current approach should be tossed. However, that is a future step. There is a learning process involved, and incremental improvements to your current code is one way to get there.
First, I'd like to get rid of the C-style memory management. Most of the time, using calloc in C++ code is wrong. Let's replace the raw pointer with a smart pointer. A shared_ptr looks like it will help the process along.
// Instead of a raw pointer to void, use a smart pointer to void.
std::shared_ptr<void> args;
// Use C++ memory management, not calloc.
args = std::make_shared<args_st<int,float>>();
// or
args = std::make_shared<args_st<float,float>>();
This is still not great, as it still uses a pointer to void, which is rarely needed in C++ code unless interfacing with a library written in C. It is, though, an improvement. One side effect of using a pointer to void is the need for casts to get back to the original type. This should be avoided. I can address this in your code by defining correctly-typed variables inside the if statement. The args variable will still be used to hold your pointer once the correctly-typed variables go out of scope.
More improvements along this vein can come later.
The key improvement I would make is to use the functional std::function instead of a function pointer. A std::function is a generalization of a function pointer, able to do more albeit with more overhead. The overhead is warranted here in the interest of robust code.
An advantage of std::function is that the parameter to g() does not need to be known by the code that invokes the std::function. The old style of doing this was std::bind, but lambdas provide a more readable approach. Not only do you not have to worry about the type of args when it comes time to call your function, you don't even need to worry about args.
int someFunction() {
// Use a smart pointer so you do not have to worry about releasing the memory.
std::shared_ptr<void> args;
// Use a functional as a more convenient alternative to a function pointer.
// Note the lack of parameters (nothing inside the parentheses).
std::function<void()> func;
if ( /* some runtime condition */ ) {
// Start with a pointer to something other than void.
auto real_args = std::make_shared<args_st<int,float>>();
// An immediate function call:
f(real_args.get());
// Choosing a function to be called later:
// Note that this captures a pointer to the data, not a copy of the data.
// Hence changes to the data will be reflected when this is invoked.
func = [real_args]() { g(real_args.get()); };
// It's only here, as real_args is about to go out of scope, where
// we lose the type information.
args = real_args;
}
else {
// Similar to the above, so I'll reduce the commentary.
auto real_args = std::make_shared<args_st<float,float>>();
func = [real_args]() { g(real_args.get()); };
args = real_args;
}
/* other code that does stuff with args */
/* This code is probably poor C++ style, but that can be addressed later. */
// Invoke the function.
func();
return 0;
}
Your next step probably should be to do some reading on these features so you understand what this code does. Then you should be in a better position to leverage the power of C++.
I have a legacy C code base, which I am migrating to C++ in a piecemeal fashion. It includes an interpreter, so there is a need to wrap static functions and arguments for use by the interpreter. So a typical function for export to the interpreter may have the following signature:
static void do_strstr(struct value * p)
and be exposed to the interpreter like so:
using vptr = void (*) ();
template <typename Func>
constexpr vptr to_vptr(Func && func)
{ return reinterpret_cast<vptr>(func); }
struct function string_funs[] = {
...
{ C_FN3, X_A3, "SSI", to_vptr(do_strstr), "find" },
...
};
This has been proven to work. The drawback with the method so far is that the called function must allocate memory onto a temporary stack. An improvement would be where the called function just returns a string, for example. This function is then wrapped, where the wrapper does the memory magic behind the scenes. This allows functions to created in a more vanilla way.
Here is an implementation which concatenates two strings using my improved method:
static std::string do_concata(struct value* p)
{
std::string s1 = (p)->gString();
std::string s2 = (p+1)->gString();
return s1+s2;
}
I create a helper function:
static void do_concata_1(struct value* p)
{
wrapfunc(do_concata)(p);
}
where the somewhat generic wrapper is defined as:
std::function<void(struct value*)>
wrapfunc(std::function<std::string(struct value*)> func)
{
auto fn = [=](struct value* p) {
std::string s = func(p);
char* ret = alloc_tmp_mem(s.size()+1);
strcpy(ret, s.c_str());
p->sString(ret);
return;
};
return fn;
}
which is exposed to the interpreter as follows:
struct function string_funs[] = {
...
{ C_FN2, X_A2, "SS", to_vptr(do_concata_1), "concata" },
...
};
I am not satisfied with this solution, though, as it requires a helper function for each function I define. It would be better if I could eliminate do_concata_1 and write another function that wraps the wrapfunc.
And this is where the problem is. If I write:
vptr to_vptr_1(std::function<void(struct value*)> func)
{
return to_vptr(wrapfunc(func));
}
then the compiler complains:
stringo.cc: In function ‘void (* to_vptr_1(std::function<void(value*)>))()’:
stringo.cc:373:30: error: could not convert ‘func’ from ‘std::function<void(value*)>’ to ‘std::function<std::__cxx11::basic_string<char>(value*)>’
return to_vptr(wrapfunc(func));
which is bizarre in my mind, because where did the std::__cxx11::basic_string<char> come from? It should be void, surely?
I'm at a loss to see what the fix should be. I am also a bit confused as to whether I should be passing copies of functions, references to functions, or the enigmatic && r-vale references.
In to_vptr_1(), func is established as a function that returns void. But func is passed to wrapfunc(), which expects a function that returns std::string. The compiler does not have a way to convert func from std::function<void(struct value*)> to std::function<std::string(struct value*)>, so it emits the error message.
reinterpret_cast from std::function to raw function pointer is not going to work. This question has some good discussion on the topic, and this one has a solution that could perhaps be reworked for this situation.
I have a tuple in D. I want to apply an element-wise operation on that tuple, and get that transformed tuple for passing into another function that accepts variadic template arguments. The execution path of the transform is defined at compile time, but the actual value is not.
The purpose of this is similar to the template mechanism used in C++'s bind construct, for determining where to use placeholders/passed arguments and where to use stored arguments at compile time.
How do I accomplish this?
this is the first time in D I've ever missed a feature in C++11's template system: the pack/unpack operator - please make me not feel bad :-(
EDIT: Ended up using mixins, because apparently any generic programming solution you want can be solved by using them. May answer with them if no one comes up with anything more elegant than taking D's ridiculously powerful jackhammer-of-a-generic-programming-tool to it.
The element of a tuple can be anything that a template alias parameter can be. However, run-time expressions cannot be alias parameters - they are evaluated at compile time. Thus, it is not possible to transform a tuple using a transformation that runs at compile-time (barring workarounds such as where the transformation defines a #property function that returns the result).
If the expression and transformation can be evaluated at compile-time, see staticMap from std.typetuple.
If I understand the question right, then this is possible but it's a highly experimental (undocumented and not guaranteed to always work) feature:
import std.stdio;
import std.traits;
import std.typetuple;
ReturnType!Call Delay(alias Call, alias arg)() { return Call(arg); }
template Map(alias Call, args...)
{
static if (args.length > 1)
alias Map = TypeTuple!(Delay!(Call, args[0]),Map!(Call, args[1..$]));
else
alias Map = Delay!(Call, args[0]);
}
int square(int arg)
{
return arg * arg;
}
void print(int res1, int res2)
{
writefln("%s %s", res1, res2); // writes '25 100'
}
void test(Args...)(Args args)
{
print(Map!(square, args));
}
void main()
{
int x = 5;
int y = 10;
test(x, y);
}
Originally asked here: Mapping variadic template arguments in D
I have exported functions with quite a bit of boilerplate, and am attempting to use string mixins to help hide the mess and sugar it up. The problem is that I have no idea how I could pass an anonymous function into the string mixin. I'd like to avoid writing the function as a string if at all possible.
// The function that the anonymous function below ultimately gets passed to.
char* magic(F...)(string function(F) func) { ... }
string genDcode(string name, alias func)() {
return xformat(q{
extern(C) export char* %s(int blah) {
// What would I inject into the string in place of 'func'
// in order to call the 'func' passed into the template?
return magic(func);
}
}, name);
}
// Calls a function to generate code to mix into the global scope.
// The anonymous function must allow abritrary parameters.
mixin(genDcode!("funcName", function(string foo, float bar) {
return "Herpderp";
}));
This is of course not the full picture, and most of the boilerplate is trimmed, but it's enough to show the problem. I've thought about injecting the function pointer as an int, and casting back into a callable type, but unsurprisingly, you can only get the function pointer at runtime.
I've tried mixin templates, which elimitates the function passing problem, but the linker cannot seem to find export functions generated from such mixins. They appear to have some extra qualifiers, and I can't use a dot in the DEF file.
Old question, but a relatively new feature might help solve it: D now has a pragma(mangle) which you can put inside a mixin template to force a particular name for the linker:
mixin template genDcode(string name, alias func) {
// pragma mangle is seen by the linker instead of the name...
pragma(mangle, name) extern(C) export char* impl(int blah) {
return magic(func);
}
}
char* magic(F...)(string function(F) func) { return null; }
mixin genDcode!("funcName", function(string foo, float bar) {
return "Herpderp";
});
I'm trying to send D's sort function as a template argument to the pipe function. When I use sort without template arguments it works:
import std.stdio,std.algorithm,std.functional;
void main()
{
auto arr=pipe!(sort)([1,3,2]);
writeln(arr);
}
However, when I try to use sort with a template argument:
import std.stdio,std.algorithm,std.functional;
void main()
{
auto arr=pipe!(sort!"b<a")([1,3,2]);
writeln(arr);
}
I get an error - main.d(5): Error: template instance sort!("b<a") sort!("b<a") does not match template declaration sort(alias less = "a < b",SwapStrategy ss = SwapStrategy.unstable,Range)
Why does it happen? sort!"b<a" works on it's own, and it has the same arguments and return types as sort, so why does pipe accept sort but not sort!"b<a"? And is there a correct syntax for what I try to do?
UPDATE
OK, I've tried to wrap the sort function. The following code works:
import std.stdio,std.algorithm,std.functional,std.array;
template mysort(string comparer)
{
auto mysort(T)(T source)
{
sort!comparer(source);
return source;
}
}
void main()
{
auto arr=pipe!(mysort!"b<a")([1,3,2]);
writeln(arr);
}
So why doesn't the original version work? is this because of the extra template parameters sort takes?
Yes it's because of the extra template parameters — specifically the Range parameter. The problem can be reduced to
size_t sort2(alias f, Range)(Range range)
{
return 0;
}
alias sort2!"b<a" u;
The instantiation sort!"b<a" will fail because the Range is not determined. The function call sort2!"b<a"([1,2,3]) works because the parameter [1,2,3] can tell the compiler the type Range is int[]. This is known as "implicit function template instantiation (IFTI)". But IFTI only works when it is used as a function. In your use case, sort!"b<a" is instantiated without providing all parameters, thus the error.
This can be fixed by making the input a function literal, which is just similar to your mysort solution:
auto arr = pipe!(x => sort!"b<a"(x))([1,3,2]);
Or you could provide all required template parameters. This makes the code very unreadable though.
auto arr = pipe!(sort!("b<a", SwapStrategy.unstable, int[]))([1,3,2]);