ppr of HsDo

[Alan & Kim Zimmerman]

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 <alan.zimm at gmail.com>
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 at 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 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
>>
>>
>>
>> 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 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
>>
>>
>>
>> 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 at haskell.org] *On Behalf Of *Alan
>> & Kim Zimmerman
>> *Sent:* 10 November 2016 07:01
>> *To:* ghc-devs at 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
>>
>>
>>
>
>
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