[GHC] #13303: Bad pretty printer for let bindings in a do-notation with braces

[GHC]

#13303: Bad pretty printer for let bindings in a do-notation with braces
-------------------------------------+-------------------------------------
           Reporter:  rem            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:  do-notation,   |  Operating System:  Unknown/Multiple
  let, parse                         |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Let bindings in do-notations with braces like this:

 {{{#!hs
 module Thing where

 thing = do {let {x = 1};
             print x}
 }}}

 get pretty printed with the braces around the bindings stripped, like
 this:

 {{{#!hs
 module Thing where

 thing = do {let x = 1;
             print x}
 }}}

 Steps to reproduce: copy the module into Try.hs then call: ghc -ddump-rn
 Try.hs

 I traced the code to:
 https://github.com/ghc/ghc/blob/e28fbbb7c3d5904a88b4743d0d10f212d61d8293/compiler/hsSyn/HsExpr.hs#L1929
 which points to the pretty printer for list of declarations:
 https://github.com/ghc/ghc/blob/95dc6dc070deac733d4a4a63a93e606a2e772a67/compiler/hsSyn/HsBinds.hs#L492
 Indeed there is a comment on the two ways to pretty print binders -
 without braces in multiple lines and with braces in one line. However the
 comment does not explain the choice, and as discussed in #13302 the former
 won't parse in a do-notation.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13303>
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