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digitalmars.D - switch case alternative

reply "seany" <seany uni-bonn.de> writes:
Hello,

I have the following problem.

In the input consol I can input a string, and the system will 
take action based on such.

So if I input add_2_with_2, it will give me a 4, if I input 
sqrt_4 it will give me 2, etc. Generally, you will do this with a 
switch / case command, but the problem is, then I need a 
condition for each case. So if i want to ADITTIONALLY input 
cube_2, then I have to write a case for this.

However, I would like to do the same without having to explicitly 
write a case each time I insert a new command. So forexample, if 
in input "FUNCTION_1", then the program should look in a specific 
place, in a specific forlder / file, find out if the function is 
defined, and execute it. If not defined in the file / folder, 
then it should throw AN EXCEPTION. If I additionally want to 
input "FUNCTION_2", then i will define the function in the same 
file or folder (whatever is possible with D) and then let the 
original program to automatically search and execute.

Can this be done in D?

(sorry for stupid question and bad English)
Nov 02 2013
parent "TheFlyingFiddle" <theflyingfiddle gmail.com> writes:
On Saturday, 2 November 2013 at 18:19:43 UTC, seany wrote:

 Hello,

 I have the following problem.

 In the input consol I can input a string, and the system will 
 take action based on such.

 So if I input add_2_with_2, it will give me a 4, if I input 
 sqrt_4 it will give me 2, etc. Generally, you will do this with 
 a switch / case command, but the problem is, then I need a 
 condition for each case. So if i want to ADITTIONALLY input 
 cube_2, then I have to write a case for this.

 However, I would like to do the same without having to 
 explicitly write a case each time I insert a new command. So 
 forexample, if in input "FUNCTION_1", then the program should 
 look in a specific place, in a specific forlder / file, find 
 out if the function is defined, and execute it. If not defined 
 in the file / folder, then it should throw AN EXCEPTION. If I 
 additionally want to input "FUNCTION_2", then i will define the 
 function in the same file or folder (whatever is possible with 
 D) and then let the original program to automatically search 
 and execute.

 Can this be done in D?

 (sorry for stupid question and bad English)


It's not a bad qustion! However plz use the Dlang.lern subform 
for posts like this.

To do what you want to do compile time reflection comes to mind. 
The code
below implements the essentials you need to get such a system 
working.

In a file (lets call it functions.d) declare all the functions 
that you want your framework to be able to call. Then at compile 
time place all functions of a specific type in an assosiative 
array (dictionary), the code below does this whenever 
buildFuncDictionary is called.

//In functions.d
module functions;
import std.math;

//Some Unary functions
real square(real a) { return a * a; }
real sqrt(real a) { return std.math.sqrt(a); }
real cube(real a) { return a * a * a; }

//Some binary functions
real add(real a, real b) { return a + b; }
real sub(real a, real b) { return a - b; }
real mult(real a, real b) { return a * b; }
real div(real a, real b) { return a / b; }

module math;
import functions;

alias real function(real) UnaryNumberFunc;
enum unaryNumberFunctions = buildFuncDict!(UnaryNumberFunc, 
functions);

alias real function(real, real) BinaryNumberFunc;
enum binaryNumberFunctions = buildFuncDict!(BinaryNumberFunc, 
functions);

//Builds an AA at compile time over all avalible functions from
//The specified module of the Type specified.
FuncType[string] buildFuncDict(FuncType, alias module_)()
{
     import std.typetuple;
     FuncType[string] functions;
     foreach(member; __traits(allMembers, module_)) {
	//Member is just a string. This gets the actual function.
	//We use a typetuple since this is the only way to store the
	//alias member. (I think)
	alias TypeTuple!(__traits(getMember, module_, member)) 
actualMember;
		
	static if(is(typeof(&actualMember[0]) == FuncType)) {
	    //Gets the name of the function;
	    string name = __traits(identifier, actualMember[0]);
	    //Takes the function and places it in the dictionary.
             functions[name] = &actualMember[0];
         }
     }

     return functions;
}

void main()
{
    import std.stdio;
	
    writeln(unaryNumberFunctions["square"](10));    //Prints 100
    writeln(binaryNumberFunctions["add"](10, 10)); //Prints 20     
             writeln(binaryNumberFunctions["sub"](10,5));	  
//Prints 5;

     readln();
}
Nov 02 2013