## digitalmars.D - Partial argument specification

- Lars Kyllingstad (37/37) Oct 17 2008 Hello,
- Lutger (1/1) Oct 17 2008 I think this is possible using std.bind
- Denis Koroskin (31/69) Oct 17 2008 You *have to* store an "int i" parameter somewhere unless you want to be...
- Bill Baxter (59/59) Oct 17 2008 T24gRnJpLCBPY3QgMTcsIDIwMDggYXQgODo1MSBQTSwgRGVuaXMgS29yb3NraW4gPDJrb3Jk...
- Robert Fraser (2/108) Oct 17 2008 I think all he was asking for is currying, which is indeed possible.
- Bill Baxter (73/73) Oct 17 2008 T24gU2F0LCBPY3QgMTgsIDIwMDggYXQgNzowNSBBTSwgUm9iZXJ0IEZyYXNlcgo8ZnJhc2Vy...
- Lars Kyllingstad (11/138) Oct 18 2008 I didn't know what currying was, so I checked out the Wikipedia article....
- downs (6/66) Oct 17 2008 How about an implicit functor?
- Lars Kyllingstad (7/78) Oct 18 2008 Ok, but say I want to do this several times:
- downs (3/89) Oct 20 2008 Yes, but how could it possibly do that?
- Lars Kyllingstad (18/107) Oct 20 2008 I'm not sure I understand. Wouldn't it be possible to do something like

Hello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real) Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think? -Lars

Oct 17 2008

Lars Kyllingstad <public kyllingen.nospamnet> писал(а) в своём письме Fri, 17 Oct 2008 15:27:52 +0400:Hello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real)You *have to* store an "int i" parameter somewhere unless you want to be a compile time constant. Use a template in this case. Alternatively use a struct or class wrapper. Struct won't allocate heap, class gives more flexibility. Delegates allow contruction on-fly without (even more flexibility) at the cost of additional overhead. I'd go with delegates unless the performance degrade drastically. // use of delegates: creal delegate(real) f2 = (real x){ return f(x, 2); } real x = ...; creal result = f2(x); // same as f(x, 2); // use of a class wrapper: class F { this(int i) { this.i = i; } creal opCall(real x) { return f(x, i); } private int i; } F f2 = new F(2); real x = ...; creal result = f2(x); // same as f(x, 2); // struct wrapper struct F2 { int i; creal opCall(real x) { return f(x, i); } } F2 f2 = { 2 }; real x = ...; creal result = f2(x);Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think? -Lars

Oct 17 2008

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Oct 17 2008

Bill Baxter wrote:On Fri, Oct 17, 2008 at 8:51 PM, Denis Koroskin <2korden gmail.com> wrote:I think all he was asking for is currying, which is indeed possible.Lars Kyllingstad <public kyllingen.nospamnet> () ϣ Fri, 17 Oct 2008 15:27:52 +0400:Sounds like what you want is partial evaluation (http://en.wikipedia.org/wiki/Partial_evaluation) but done at run time. That's only going to be possible if there's a compiler built into the runtime. So not possible currently. --bbHello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real)You *have to* store an "int i" parameter somewhere unless you want to be a compile time constant. Use a template in this case. Alternatively use a struct or class wrapper. Struct won't allocate heap, class gives more flexibility. Delegates allow contruction on-fly without (even more flexibility) at the cost of additional overhead. I'd go with delegates unless the performance degrade drastically. // use of delegates: creal delegate(real) f2 = (real x){ return f(x, 2); } real x = ...; creal result = f2(x); // same as f(x, 2); // use of a class wrapper: class F { this(int i) { this.i = i; } creal opCall(real x) { return f(x, i); } private int i; } F f2 = new F(2); real x = ...; creal result = f2(x); // same as f(x, 2); // struct wrapper struct F2 { int i; creal opCall(real x) { return f(x, i); } } F2 f2 = { 2 }; real x = ...; creal result = f2(x);Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think?

Oct 17 2008

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Oct 17 2008

Bill Baxter wrote:On Sat, Oct 18, 2008 at 7:05 AM, Robert Fraser <fraserofthenight gmail.com> wrote:I didn't know what currying was, so I checked out the Wikipedia article. I quote: Intuitively, currying says "if you fix some arguments, you get a function of the remaining arguments". This is exactly what I was looking for. There seems to be agreement here that this can't be done without at least a little overhead, but I still think it would be nice with a simple, intuitive syntax like the one I described. Also, I think it would fit in nicely with D2's focus on functional programming. -LarsBill Baxter wrote:You mean this kind of solution : http://web.mit.edu/d_v1.020/html/d/template.html (search for "Curry" on the page) ? No, he specifically said he didn't want the overhead of carrying around the arguments. That's basically the "functor" solution he mentions. Only it returns a delegate to the functor's method rather than returning the functor itself.On Fri, Oct 17, 2008 at 8:51 PM, Denis Koroskin <2korden gmail.com> wrote:I think all he was asking for is currying, which is indeed possible.Lars Kyllingstad <public kyllingen.nospamnet> () ϣ Fri, 17 Oct 2008 15:27:52 +0400:Sounds like what you want is partial evaluation (http://en.wikipedia.org/wiki/Partial_evaluation) but done at run time. That's only going to be possible if there's a compiler built into the runtime. So not possible currently. --bbHello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real)You *have to* store an "int i" parameter somewhere unless you want to be a compile time constant. Use a template in this case. Alternatively use a struct or class wrapper. Struct won't allocate heap, class gives more flexibility. Delegates allow contruction on-fly without (even more flexibility) at the cost of additional overhead. I'd go with delegates unless the performance degrade drastically. // use of delegates: creal delegate(real) f2 = (real x){ return f(x, 2); } real x = ...; creal result = f2(x); // same as f(x, 2); // use of a class wrapper: class F { this(int i) { this.i = i; } creal opCall(real x) { return f(x, i); } private int i; } F f2 = new F(2); real x = ...; creal result = f2(x); // same as f(x, 2); // struct wrapper struct F2 { int i; creal opCall(real x) { return f(x, i); } } F2 f2 = { 2 }; real x = ...; creal result = f2(x);Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think?

Oct 18 2008

Lars Kyllingstad wrote:Hello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real) Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think? -LarsHow about an implicit functor? creal f(real x, int i); auto f2 = bind(&f, _0, 2); // I think auto f2 = &f /rfix/ 2; // tools version This creates a functor on the heap, and is indeed slower, but it's more elegant than you make it sound :)

Oct 17 2008

downs wrote:Lars Kyllingstad wrote:Ok, but say I want to do this several times: for (real y=yStart; y<=yEnd; y+=yStep) derivative(&f /rfix/ y, 1.23); Would it be any faster if your /rfix/ (or a similar function) created the functor on the stack instead? -LarsHello, There is a feature I would very much like to see in D. I don't know if it has been discussed before, or whether is's even possible, but I'm just going to throw it out here. Please tell me what you think. Suppose you have a function that takes a certain number of arguments, say creal f(real x, int i); Then it would be neat if one could specify just some of the arguments, and have the result be a pointer to a function that takes the remaining arguments. To clarify, the type of f(real, 2) would then be creal function(real) Why would this be nice? As an example, say you have a function that calculates the derivative of another function at a certain point: real derivative(real function(real), real z); With the above notation I can use this for functions of several variables: real f(real x, real y) { ... }; auto dfdx = derivative( f(real, 1.23), 4.56 ); As an added bonus, I can even differentiate with respect to y: auto dfdy = derivative( f(1.23, real), 4.56 ); Already, there are several ways to do similar things, but in my opinion they are not as good: 1. Use templates Nice, but only works when the pre-specified arguments are known at compile time. (Or is there some trick I don't know about?) 2. Use functors This works, but leads to worse performance and is in my opinion less elegant. One has to type a lot of code just to define simple functions. 3. Use wrapper functions Same problems as (2), and also leads to use of global variables. 4. The GSL way: Pass remaining arguments in a void* pointer. Example: real derivative(real function(real, void*), real z); IMO, this is UGLY, not to mention un-D-ish. I mainly use D for numerical computations, hence the examples above. But I'm sure there are many other uses for such a feature. What do you think? -LarsHow about an implicit functor? creal f(real x, int i); auto f2 = bind(&f, _0, 2); // I think auto f2 = &f /rfix/ 2; // tools version This creates a functor on the heap, and is indeed slower, but it's more elegant than you make it sound :)

Oct 18 2008

Lars Kyllingstad wrote:downs wrote:Yes, but how could it possibly do that? After all, then the data would become invalid on scope exit, which is exactly what we're trying to avoid!Lars Kyllingstad wrote:Ok, but say I want to do this several times: for (real y=yStart; y<=yEnd; y+=yStep) derivative(&f /rfix/ y, 1.23); Would it be any faster if your /rfix/ (or a similar function) created the functor on the stack instead? -Lars

Oct 20 2008

downs wrote:Lars Kyllingstad wrote:I'm not sure I understand. Wouldn't it be possible to do something like this: struct FunctionWrapper { ... SomeType opCall(...) { ... } } and then have /rfix/ (or bind) return a FunctionWrapper? Of course it would have to return by value instead of returning a pointer, since whatever is pointed to will be lost on scope exit. If there are no pointers to stack data inside the struct there shouldn't be a problem. Don't know if it's possible to avoid this, though. But perhaps the cost of returning a (possibly large) struct by value would outweigh the cost of calling functors on the heap? I guess it depends on how many times a single functor is called, compared to how often a new one is created. -Larsdowns wrote:Yes, but how could it possibly do that? After all, then the data would become invalid on scope exit, which is exactly what we're trying to avoid!

Oct 20 2008