Re: ppr of HsDo

As a follow up, I will be continuing with the least-invasive change, which is to keep the existing braces/semis, and make sure that they are all produced correctly. Alan On Thu, Nov 10, 2016 at 12:06 PM, Alan & Kim Zimmerman wrote:

For context, I am putting in a test suite similar to the one for ghc-exactprint to ensure that the pretty printer always generates code that can be round tripped back to the original AST.

This means that fears of some uncaught case requiring us do it the guaranteed safe way should be allayed.

In the process I am updating the pretty printer.

So the question really is, given the existence of that test suite, what style of code should we have in our messages, and in pretty printed code?

Alan

On Thu, Nov 10, 2016 at 12:02 PM, Simon Peyton Jones < simonpj@microsoft.com> wrote:

...

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!

Simon

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

Thanks.

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.

Alan

On Thu, Nov 10, 2016 at 10:24 AM, Simon Peyton Jones < simonpj@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

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

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

foo = 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!

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

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

SImon

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

The pretty printer turns

foo = do let x = 1 Just 5

into

foo = 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

foo = do let x = 1 Just 5

but I am not sure if there is a parser bug too.

Alan