id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4886	"GHC ""extensions"" for (pre/post)processors"	pumpkin		"I've been using Conor McBride's SHE recently and while it's excellent, error messages produced by code it generates are generally unreadable. This is because the code preprocessor has no opportunity to insert an error post-processor.

What if, instead of SHE being a separate process that GHC ran (and is thus not automatically cabal-installable and so on) on each source file, GHC had a simple extensions interface, where an extension would be a module that contained two functions and a type:

{{{
data State = ...

preprocess :: String -> (String, State)
postprocess :: String -> State -> String
}}}

GHC would take care of running the preprocessor before doing its own thing, and would preserve the State and pass it back to postprocess the error messages.

This would allow me to write something like {-# EXTENSION Extension.SHE #-} and have it magically produce readable error messages (assuming someone wrote an error postprocessor for SHE).

Of course, since the error messages don't really have a structured format, there'd be no way for these things to compose, since a postprocessor could only make assumptions about error messages it already knows (and not those generated by other postprocessors). But this seems like a fairly low-resistance way to test new features and have them actually be usable without having to know the internals of the desugaring in the preprocessor."	feature request	closed	normal		Compiler	7.0.1	wontfix			Unknown/Multiple	Unknown/Multiple	None/Unknown