Re: [Haskell-cafe] RFC: rewrite-with-location proposal

Hi Evan, On Wed, Feb 04, 2015 at 04:10:56PM -0800, Evan Laforge wrote:

On Wed, Feb 4, 2015 at 3:51 PM, Eric Seidel wrote:

...

Yep, my original motivation was getting access to source locations within embedded DSLs. The call-stack is a nice and easy extension, but I'm not sure how useful it will be in practice, as the first function that doesn't request a CallStack parameter will cut off the stack. This means that the generated stacks will often be quite short, I imagine.

Well, as I said, all I really care about is the direct caller. From the example in the commit, it looks like the function with the (?x :: Location) annotation can get its immediate caller, even if that caller doesn't have the annotation. If that's true, that's all that is needed!

I completely agree with you. Logging and failing test cases where my main motivation when I looked at the problem domain (even though I still think the situation with error/undefined is unfortunate too).

And from my point of view, it's not just "maybe useful in practice", but absolutely required, to the point where I wrote a custom preprocessor for it. I've been using it for 6 or 7 years and I sort of forgot that other people don't have it. I actually have no idea how other people do logging... just hope the message is unique and grep -n all the time? And for tests, manually give every single assertion a unique name and grep -n again?

hspec-discover adds heuristic source locations by now (without really parsing any Haskell code, so it's pretty robust, worst case is you don't get a source location). This is on a spec item (== test case) granularity. I still want proper source locations, so that we can attach them to individual expectations/assertions.

Enable TH globally?

For logging-facade I added a TH version, but I don't really like it. And again, I'm super eager to use proper source locations here, once the patch got merged!

I don't think it needs to be used at all in the standard libraries, since logging and testing are not part of base. I can understand if the merge window for 7.10 is closed, but trying to come up with a use in base shouldn't hold it up!

As you argued in a later mail, I would be in favor of using it for all partial functions in base, too (but we may want to spend some time looking at how list fusion may be affected). But I think it makes sense to treat this as two separate discussions. Hopefully we get this patch merged ASAP, then we can still discuss to what extend we want to use it in base. Cheers, Simon [1] http://hackage.haskell.org/package/logging-facade-0.0.0/docs/System-Logging-...