GHC not parsing TH pretty printed code

[Simon Peyton Jones]

Richard is spot on.

Meanwhile, do open a ticket on this.  Maybe someone else will jump in to help.

Do give a step-by-step way to reproduce the problem.

Simon

| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
| Richard Eisenberg
| Sent: 09 May 2014 05:25
| To: George Roldugin
| Cc: ghc-devs at haskell.org
| Subject: Re: GHC not parsing TH pretty printed code
| 
| Well, the problem isn't quite that the semicolon is expected before the
| second line -- it's that the semicolon is interpreted as separating let-
| definitions, not do-statements.
| 
| For example, the following works:
| 
| bar = do {let {x = 5};
|           return x}
| 
| Looking quickly through the TH's pretty-printer code, there seem to be
| several places where problems like this might arise, due to lack of
| braces or too many of them.
| 
| What to do? If you're blocked by this issue, just edit the pretty-
| printer code. The code is actually quite straightforward. Get the
| sources
| (https://ghc.haskell.org/trac/ghc/wiki/Building/GettingTheSources),
| prep your machine
| (https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation), and build.
| Given that TH has changed since the 7.8 release, I recommend saying
| `sync-all checkout ghc-7.8` to get the 7.8 branch before building.
| 
| After building, do some sanity checks to make sure your hand-built GHC
| is working.
| 
| Then, just edit libraries/template-haskell/Language/Haskell/TH/Ppr.hs
| until things work. In particular, I bet adding a call to `braces` on
| line 150 will go a long way here.
| 
| Even if we fixed the bug you reported, that bugfix would be in HEAD,
| where TH is different than in 7.8. It's possible it would be merged for
| 7.8.3, but my guess is that you don't want to wait that long.
| 
| I hope this helps!
| Richard
| 
| On May 9, 2014, at 12:08 AM, George Roldugin <groldugin at cse.unsw.edu.au>
| wrote:
| 
| > Hi all,
| >
| > I've hit a problem with GHC 7.8 where the code pretty printed by
| Template Haskell fails to parse.
| >
| > The problem is with the placement of explicit semicolons in place of
| indentation.
| >
| > The following code pretty printed by TH won't parse:
| >
| > bar = do {let x = 5;
| >          return x}
| >
| >
| > GHC expects the following placement of semicolons:
| >
| > bar = do {let x = 5
| >         ; return x}
| >
| >
| > I reported this bug and included sample TH code to reproduce the
| problem here:
| > https://ghc.haskell.org/trac/ghc/ticket/9022
| >
| > I rely on compiling code constructed with TH for my thesis project.
| >
| > Is there a work around I could use besides going back to GHC 7.6?
| >
| > Cheers,
| > George
| > _______________________________________________
| > ghc-devs mailing list
| > ghc-devs at haskell.org
| > http://www.haskell.org/mailman/listinfo/ghc-devs
| 
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