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

[Michael Snoyman]

Quite a while back, Simon Hengel and I put together a proposal[1] for a new
feature in GHC. The basic idea is pretty simple: provide a new pragma that
could be used like so:

error :: String -> a
errorLoc :: IO Location -> String -> a
{-# REWRITE_WITH_LOCATION error errorLoc #-}

Then all usages of `error` would be converted into calls to `errorLoc` by
the compiler, passing in the location information of where the call
originated from. Our three intended use cases are:

* Locations for failing test cases in a test framework
* Locations for log messages
* assert/error/undefined

Note that the current behavior of the assert function[2] already includes
this kind of approach, but it is a special case hard-coded into the
compiler. This proposal essentially generalizes that behavior and makes it
available for all functions, whether included with GHC or user-defined.

The proposal spells out some details of this approach, and contrasts with
other methods being used today for the same purpose, such as TH and CPP.

Michael

[1] https://github.com/sol/rewrite-with-location
[2]
http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Control-Exception.html#v:assert
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