[Haskell-cafe] Printing call site for partial functions

[Evan Laforge]

> And then have the compiler automatically include (optional) package
> name, module name, and line number where `headContext` was called. How
> about we borrow a bit from rewrite rules, and have a pragma such as:
>
>    {-# WITH_CONTEXT head headContext #-}

This seems similar to the SRCLOC_ANNOTATE pragma supported by jhc.

I'd love to have this feature.  I'd just like to point out it's not
just useful for partial functions like 'head', but also for logging
and "expected" exceptions like Left.  It's also extremely useful for
tests, when I get "check failed: 3 /= 4" it's really important to see
filename and line number.

There are various hacks out there to get this, but they all involve
annotating the call sites with some magic, either an 'assert' (and
then runtime overhead on every function that uses it) or a TH splice
(and then you have to turn TH on in every module).  I have literally
thousands of lines of tests that would all need an extra boilerplate
argument.

I implemented a preprocessor that is effectively like SRCLOC_ANNOTATE
and it works ok for me, but it's not a good general solution because
it's hardcoded to certain symbols and slows down compilation too much.
 And being specific to my app of course it wouldn't help in your case
:)

But it won't happen unless one of us few people who care about it just
goes and implements it.  I don't think it's much of a priority with
others.