February 28, 2007
I have been writing a compile-time parser using Tuples and CTFE.  I admit, I am writing it basically by thinking iteratively, and writing templates that execute the code that would have otherwise happened at runtime.

There is one big annoyance I keep running into, though, and it is the inability to "return" from this sort of template in the middle.  Imagine that I'm thinking something like this (writing it as runtime code, first):
  parse_tree parse(char[] str) {
    if(str.length == 0)
      return new parse_tree; // empty tree

    // do lots of other complex stuff
  }

Now, when I translate this into templates, I get something like:
  template Parse(char[] str) {
    static if(str.length == 0)
      alias struct {} Parse;
    else {
      // do lots of other complex stuff
    }
  }

Sometimes, I end up pretty deeply nested in the else clauses of my static if's.  It's technically correct, but ugly.  What I would like to see is the template equivalent of "return" - something which would break you out of the current evaluation.  My idea is to use the syntax "break template;"

Then the template code becomes something which I think is clearer, and closer to how I'm actually thinking about things:
  template Parse(char[] str) {
    static if(str.length == 0) {
      alias struct {} Parse;
      break template;
    }

    // do lots of other complex stuff
  }

Russ
February 28, 2007
Russell Lewis wrote:
> I have been writing a compile-time parser using Tuples and CTFE.  I admit, I am writing it basically by thinking iteratively, and writing templates that execute the code that would have otherwise happened at runtime.
> 
> There is one big annoyance I keep running into, though, and it is the inability to "return" from this sort of template in the middle.  Imagine that I'm thinking something like this (writing it as runtime code, first):
>   parse_tree parse(char[] str) {
>     if(str.length == 0)
>       return new parse_tree; // empty tree
> 
>     // do lots of other complex stuff
>   }
> 
> Now, when I translate this into templates, I get something like:
>   template Parse(char[] str) {
>     static if(str.length == 0)
>       alias struct {} Parse;
>     else {
>       // do lots of other complex stuff
>     }
>   }
> 
> Sometimes, I end up pretty deeply nested in the else clauses of my static if's.  It's technically correct, but ugly.  What I would like to see is the template equivalent of "return" - something which would break you out of the current evaluation.  My idea is to use the syntax "break template;"
> 
> Then the template code becomes something which I think is clearer, and closer to how I'm actually thinking about things:
>   template Parse(char[] str) {
>     static if(str.length == 0) {
>       alias struct {} Parse;
>       break template;
>     }
> 
>     // do lots of other complex stuff
>   }
> 
> Russ

I ran into this too when attempting some compile-time parsing.  While I haven't attempted anything since compile-time function evaluation (CTFE) was introduced, I did devise the following technique for templates:

template ParseTerm(){
	const char[] Reason = null; // is non-null for any term that fails, and contains the reason
	const bool Passed = true;   // is false for any term that fails
        alias Foobar Result;        // where foobar is any value that you wish to "return" from this term.
}

As long as all your templates in your parser adhere to this "interface", you can do things like this:

template ParseSomething(char[] str){
	alias ParseSomethingElse!(str) Term1;

	static if(Term1.Passed){
		alias Term1.Result Result; // return the result (could be anything really)
		const char[] Reason = null;
		const bool Passed = true;
	}
	else{
		alias Tuple!() Result; // return an 'empty' result
		const char[] Reason = "Parse failure: " ~ Term1.Reason;
		const bool Passed = false;
	}
}

... then use 'ParseSomething' in the same manner as 'ParseSomethingElse' elsewhere in your parser.

-- 
- EricAnderton at yahoo
March 01, 2007
I suggest looking at using compile time function evaluation for string parsing instead of using templates.
March 01, 2007
Walter Bright wrote:
> I suggest looking at using compile time function evaluation for string parsing instead of using templates.

I've tried that, but I need to return a struct which represents the parse information.  Thus, I can't use CTFE.
March 02, 2007
Russell Lewis wrote:
> Walter Bright wrote:
>> I suggest looking at using compile time function evaluation for string parsing instead of using templates.
> 
> I've tried that, but I need to return a struct which represents the parse information.  Thus, I can't use CTFE.

Even if CTFE is upgraded to allow for structs, that still won't solve my problem entirely.  Since I'm parsing a string into a parse tree, I need to be able to allocate an arbitrary number of nodes and use pointers or dynamic arrays in my return value.

Or, as I'm currently doing, I can dynamically generate types by nesting types inside other types.

The point is: CTFE needs to be a *lot* more flexible before it works for this sort of situation.
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