ppr of HsDo

[Alan & Kim Zimmerman]

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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