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