When isInputRange fails, it doesn't tell us why. Did you not add empty? Or misspell popFront? It just tells you it isn't a range.

I'm thinking a better way might be to write the check like this:

// compile a regular function so we get full error from the compiler...
bool checkInputRange(T)() {
    if(!__ctfe) { // don't want to *actually* run it
        T t = void;
        if(t.empty) {}
        t.popFront();
        auto f = t.front;
    }
    return true;
}

// and isInputRange is basically the same as today, but the
// helper function isn't anonymous
template isInputRange(T) {
    enum isInputRange = is(typeof(checkInputRange!T));
}



Consider this:

struct Test {
    int[] s;
    bool eampty() { return s.length == 0; }
    int front() { return s[0]; }
    void popFront() { s = s[1 .. $]; }
}
static assert(isInputRange!Test);

cte.d(33): Error: static assert  (isInputRange!(Test)) is false


We know it failed, but we don't know why. Change the assert to this:

static assert(checkInputRange!Test);

And we get a more helpful error:
cte.d(4): Error: no property 'empty' for type 'Test', did you mean 'eampty'?


Still not perfect, it points to the helper function line, but at least the "no property 'empty' for type 'Test'" is a lot more specific.

Fix the typo and it all works.



Any better ideas? I just sometimes get frustrated, especially with more complex ranges, when the duck type doesn't work and it is hard to find why. This is one idea.

On a similar vein, template constraints can lead to some ugly messages. Why didn't it match any of them? But I think this has to be a compiler change and might spam all kinds of nonsense, whereas tweaking isInputRange etc. is a fairly conservative change that I think will help a lot too.

On Thu, Feb 27, 2014 at 04:12:02AM +0000, Adam D. Ruppe wrote: [...]
> Any better ideas? I just sometimes get frustrated, especially with more complex ranges, when the duck type doesn't work and it is hard to find why. This is one idea.
> 
> On a similar vein, template constraints can lead to some ugly messages. Why didn't it match any of them? But I think this has to be a compiler change and might spam all kinds of nonsense, whereas tweaking isInputRange etc. is a fairly conservative change that I think will help a lot too.

We've talked about this before. I proposed the idea that sig constraints should be used to pick up *all* "logical" types that you want to support, and static ifs used to narrow down which subset of the accepted logical types is actually implemented for. Example:

	auto sort(R)(R range)
		if (isInputRange!R) // N.B. we accept *any* range:
				// "sort a range" is the logical
				// category we cover for.
	{
		// N.B. our implementation can only handle random access
		// ranges, so use static if.
		static if (!isRandomAccessRange!R)
			static assert(0, "Don't know how to sort a non-random access range");
		else {
			// implementation here
		}
	}

But it's not perfect, though.


T

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