[GHC] #16096: let x = ... and x = ... are not the same in GHCi

[GHC]

#16096: let x = ... and x = ... are not the same in GHCi
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        Reporter:  osa1              |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 The problem is this: in `GHCi.UI.runStmt` we use GHC's `parseStmt` to
 parse the statements. If the parser fails then we assume it's a
 declaration. For statements we do `run_stmt`, for decls we do `run_decl`.
 So effectively we compile `x = 1` as a top-level binding (a declaration)
 but `let x = 1` is compiled as if it's a `do` statement.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16096#comment:2>
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