Why is std.algorithm so complicated to use?

[Jonathan M Davis]

On Monday, July 09, 2012 22:09:54 Jacob Carlborg wrote:
> Almost every time I'm trying to use std.algorithm I run into some kind
> of error, for what seems to be fairly trivial and what one would expect
> to work. It feels like I'm constantly fighting with std.algorithm. For
> example:
> 
> import std.algorithm;
> import std.range;
> 
> struct Foo {}
> 
> auto f = Foo();
> auto foos = [f];
> auto foo = foos.map!(x => "foo");
> auto bar = foo.chain("bar");
> 
> This simple example result in the follow error:
> 
> http://pastebin.com/E4LV2UBE
> 
> Another example:
> 
> auto str = ["foo", "bar"].map!(x => x);
> auto f = str.sort();
> 
> Results in:
> 
> http://pastebin.com/BeePWQk9
> 
> I'm using DMD 2.059.

>From the looks of it (without digging into what's really going on), in both 
cases, the problem seems to be that a template constraint is insufficiently 
precise, and so it's attempting to instantiate a template when that 
instantiation should fail. If a template constraint is written correctly, it 
should be impossible for the template constraint to succeed and the template 
instantiation fail.

In general, I think that std.range and std.algorithm need better unit tests 
(ideally with every combination of range types that a particular function is 
supposed to work with being tested for that function). There are definitely 
cases where certain range types are supposed to work and don't (reference type 
ranges being a prime example). I've started on that but haven't gotten all 
that far yet. But with better testing, it should be much harder for a bad 
template constraint to be missed.

- Jonathan M Davis