ppr of HsDo

Simon Peyton Jones simonpj at microsoft.com
Thu Nov 10 10:02:31 UTC 2016

It’s not about GHC’s programming style, is it?  It’s about what the pretty-printer does.  If it were me I’d use braces and semicolons everywhere, so that I could guarantee to parse it easily.

But that’s not a strong opinion and I would willingly yield to others!


From: Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
Sent: 10 November 2016 08:31
To: Simon Peyton Jones <simonpj at microsoft.com>
Cc: ghc-devs at haskell.org
Subject: Re: ppr of HsDo

And any thoughts on my proposal to do away with the braces/semi completely?  I suspect GHC is the only significant body of code that uses that style still.

On Thu, Nov 10, 2016 at 10:24 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
I think it’s because the  “;” is treated as part of the let not part of the do.  After all, how does the implicit layout of the let know that the let-bindings are finished?

This should work

  = do { let { x = 1 };
         Just 5 }

Now the let bindings are clearly brought to an end.  Or this

  = do { let x = 1
       ; Just 5 }

Now the “’;” is to the left of the x=1 and so brings the let’s implicit layout to an end.

But not this!

  = do { let x = 1; Just 5 }

So it’s a bug in the pretty-printer, not the parser


From: ghc-devs [mailto:ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>] On Behalf Of Alan & Kim Zimmerman
Sent: 10 November 2016 07:01
To: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
Subject: ppr of HsDo

The pretty printer turns

foo = do
  let x = 1
  Just 5

  = do { let x = 1;
         Just 5 }
which does not parse, complaining about "parse error on input ‘Just’"
Is this a parser error or a ppr problem?  I am keen to fix the ppr to output

  = do let x = 1
       Just 5
but I am not sure if there is a parser bug too.

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