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

Evan Laforge writes:

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!

That's correct, though I regrettably forgot to update the Phabricator summary with the rename from Location to CallStack (the actual docs do properly talk about CallStacks). A constraint (?x :: CallStack) will always be solved for the source location that gave rise to it, when it comes from a function signature (as opposed to a use of the implicit param) you'll get the immediate call-site. Furthermore, if that call-site has a CallStack implicit param in scope, the stacks will be appended (this appending of call-stacks is the bit that I'm not sure will see much use). Does that make sense?

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? Enable TH globally? Those all seem impractical if you have or are expecting thousands of modules.

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!

You're quite right, there are plenty of reasons to want this functionality beyond error reporting, though I'd personally like to use this for `error`, `undefined`, and `assert` as well!