[GHC] #13302: Let in do-notation with braces does not parse

[GHC]

#13302: Let in do-notation with braces does not parse
-------------------------------------+-------------------------------------
        Reporter:  rem               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:  do-notation,
                                     |  let, parse
Operating System:  MacOS X           |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 I believe GHC is correct here. The non-layout syntax you have in mind is
 of course
 {{{
 do {let {x = 4}; print x}
 }}}
 In your first snippet there is no `{` after `let`, so layout comes into
 play. It seems that you expect the layout rule to insert a `}` before the
 `;`, presumably thanks to the rule
 > {{{
 >  L (t : ts) (m : ms) = } : (L (t : ts) ms)    if m ∕= 0 and parse-
 error(t)
 > }}}
 > Note 5.
 > The side condition parse-error(t) is to be interpreted as follows: if
 the tokens generated so far by L together with the next token t represent
 an invalid prefix of the Haskell grammar, and the tokens generated so far
 by L followed by the token “}” represent a valid prefix of the Haskell
 grammar, then parse-error(t) is true.
 But, even as much as
 {{{
 do {let {x = 4; print x
 }}}
 is a valid prefix of the Haskell grammar, since it could continue, for
 example,
 {{{
                         = 5}; print x}
 }}}
 So the effect of layout does not turn your first snippet into a valid
 Haskell program.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13302#comment:1>
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