patch applied (ghc): Adding a GENERATED pragma

[Simon Marlow]

Andy Gill wrote:

>  You have two files.
> 
> 012345678901234567890123456789
> Happy.y:
> 
> foo ::= foo bar     { BarBaz $1 $2 }
> 
> Happy.hs: (generated file)
> 
> [stuff]
> rhs dollar_1 dollar_2 = BarBaz dollar_1 dollar_2
> [more stuff]
> 
> Now we still want any error messages in the BarBaz code fragment to  be 
> in terms of the generated file,
> and we still want to do coverage in terms of the generated file.  After 
> all, it is a Haskell module.
> However, if we change Happy to generate the following:
> 
> Happy.hs: (generated file)
> 
> [stuff]
> rhs dollar_1 dollar_2 = {-# GENERATED "Happy.y" 1:15-1:25 #-} BarBaz  
> dollar_1 dollar_2
> [more stuff]
> 
> Under the semantics for GENERATED, this makes no difference to the  
> generated code,
> or any error messages (say BarBaz is a typo, for example). However,  Hpc 
> can use the pragma,
> and generate a coverage markup file for *both* Happy.hs (the real  
> Haskell) and Happy.y,
> where the GENERATED code is tied back to its original location.
> 
> Note that this is different than LINE, because LINE changes what the  
> current span is for
> the purposes of tagging the AST, and therefore error messages (and  code 
> coverage).
> After all, the GENERATED code is even of different length than the  
> original production code inside {}.

I'm beginning to understand, but this still seems a bit strange.

So GENERATED is only used for coverage information?  It really isn't a 
generalised LINE pragma, because it doesn't affect the locations reported in 
error messages.  I understand that this is intentional, but... hmmmm.

Cheers,
	Simon