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

[GHC]

#16096: let x = ... and x = ... are not the same in GHCi
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------
Description changed by osa1:

Old description:

> With #7253 we implemented support for `x = ...` in GHCi which is supposed
> to do the same thing as `let x = ...`, but they're currently different,
> as observed in #16089, #15721, and probably in other tickets. Here's a
> reproducer:
>
> {{{
> ~ $ ghci -ddump-bcos
> GHCi, version 8.4.4: http://www.haskell.org/ghc/  :? for help
>
> ...
>
> λ:1> let x = [True,False]
>
> ==================== Proto-BCOs ====================
> ProtoBCO ExprTopLevel_E0#0 []:
>    let sat_s1vF = ... in ...
>    bitmap:  0 []
>    PUSH_G   GHC.Types.[]
>    PUSH_G   GHC.Types.False
>    PACK     : 2
>    PUSH_L   0
>    PUSH_G   GHC.Types.True
>    PACK     : 2
>    PUSH_G   GHC.Types.[]
>    PUSH_L   1
>    PACK     : 2
>    PUSH_L   0
>    PUSH_APPLY_P
>    PUSH_G   GHC.Base.returnIO
>    SLIDE    3 3
>    ENTER
>

> λ:2> x = [True,False]
>
> ==================== Proto-BCOs ====================
> ProtoBCO x1_r1wJ#0 []:
>    GHC.Types.:
>      @ GHC.Types.Bool GHC.Types.False (GHC.Types.[] @ GHC.Types.Bool)
>    bitmap:  0 []
>    PUSH_G   GHC.Types.[]
>    PUSH_G   GHC.Types.False
>    PACK     : 2
>    ENTER
>
> ProtoBCO Ghci2.x#0 []:
>    GHC.Types.: @ GHC.Types.Bool GHC.Types.True x1_r1wJ
>    bitmap:  0 []
>    PUSH_G   x1_r1wJ
>    PUSH_G   GHC.Types.True
>    PACK     : 2
>    ENTER
>
> ...
> }}}
>
> Expected behavior: these two should generate the same byte code, and
> should be subject to same checks (e.g. for shadowing).

New description:

 With #7253 we implemented support for `x = ...` in GHCi which is supposed
 to do the same thing as `let x = ...`, but they're currently different, as
 observed in #16089, #15721, and probably in other tickets. Here's a
 reproducer:

 {{{
 ~ $ ghci -ddump-bcos
 GHCi, version 8.4.4: http://www.haskell.org/ghc/  :? for help

 ...

 λ:1> let x = [True,False]

 ==================== Proto-BCOs ====================
 ProtoBCO ExprTopLevel_E0#0 []:
    let sat_s1vF = ... in ...
    bitmap:  0 []
    PUSH_G   GHC.Types.[]
    PUSH_G   GHC.Types.False
    PACK     : 2
    PUSH_L   0
    PUSH_G   GHC.Types.True
    PACK     : 2
    PUSH_G   GHC.Types.[]
    PUSH_L   1
    PACK     : 2
    PUSH_L   0
    PUSH_APPLY_P
    PUSH_G   GHC.Base.returnIO
    SLIDE    3 3
    ENTER


 λ:2> x = [True,False]

 ==================== Proto-BCOs ====================
 ProtoBCO x1_r1wJ#0 []:
    GHC.Types.:
      @ GHC.Types.Bool GHC.Types.False (GHC.Types.[] @ GHC.Types.Bool)
    bitmap:  0 []
    PUSH_G   GHC.Types.[]
    PUSH_G   GHC.Types.False
    PACK     : 2
    ENTER

 ProtoBCO Ghci2.x#0 []:
    GHC.Types.: @ GHC.Types.Bool GHC.Types.True x1_r1wJ
    bitmap:  0 []
    PUSH_G   x1_r1wJ
    PUSH_G   GHC.Types.True
    PACK     : 2
    ENTER

 ...
 }}}

 Expected behavior: these two should generate the same byte code, and
 should be subject to same checks (e.g. for shadowing).

 (Example above uses 8.4.4, but this can be reproduced with GHC HEAD too)

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16096#comment:1>
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