[GHC] #15617: Unboxed tuples/sum error message on `a = show 5` in expression evaluation and interactive modes

[GHC]

#15617: Unboxed tuples/sum error message on `a = show 5` in expression evaluation
and interactive modes
-------------------------------------+-------------------------------------
        Reporter:  ChaiTRex          |                Owner:
                                     |  JulianLeviston
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by JulianLeviston):

 To refresh my brain (and so I don't have to re-load this into my brain
 over and over)... as per above, but with annotation:

 1. In `ghc/Main.hs`, the `main` function is the entry point for all of
 GHC.
 1. The line with `(mode, argv3, flagWarnings) <- parseModeFlags argv2`
 parses the mode flags out. There are two possible cases here: either we
 have a `Left preStartupMode` or `Right postStartupMode`. The
 `preStartupMode` case is only for doing things before GHC starts up;
 things such as outputting the version number, etc (ie not executing code).
 So, we're only interested in the `Right postStartupMode` variant.
 1. Continuing on, we start `GHC` by using `GHC.runGhc` with `mbMinusB`,
 then pull the flags out with `GHC.getSessionDynFlags`. We then `case` on
 `postStartupMode` which itself is an `Either` as well... it's similar to
 above, but here we have some `preLoadMode` values that (I'm assuming) can
 only be got when `GHC` has started up. Things like showing info, ghc
 usage, ghci usage and printing the flags. Here, though, we're only
 interested in the `Right` variant again — the `postLoadMode` value gets
 pattern matched out and;
 1. next this hits the `main'` function as: `main' postLoadMode dflags
 argv3 flagWarnings`. This function then cases on `postLoadMode`... and
 the only branches of this we're really interested in is `DoInteractive
 -> (CompManager, HscInterpreted, LinkInMemory)` and `DoEval _        ->
 (CompManager, HscInterpreted, LinkInMemory)`... there are five other
 matches, but they're make, backpack, something called MkDependHS and also
 AbiHash, then the catchall (ie `_`). Essentially we want to block
 optimisation on the interactive and eval cases. So, this matches `(mode,
 lang, link)` to the values `(CompManager, HscInterpreted, linkInMemory)`
 respectively for both of these cases (interactive and eval).
 1. Next we use a series of `let` expressions building up dflags1, dflags2,
 dflags3, etc. then, at the very end, we have another case stateament
 inside of an evaluation of `handleSourceError` to handle source code
 errors... again, the only two cases we care about are `DoInteractive` and
 `DoEval exprs`, both of which evalute to `ghciUI` expressions.
 Respectively: `ghciUI hsc_env dflags6 srcs Nothing` and `ghciUI hsc_env
 dflags6 srcs $ Just $ reverse exprs`.
 1. The `ghciUI` function pulls the flags out after initializing the
 plugins with the `hsc_env` and `dflags0`, then sets this into the session
 with `GHC.setSessionDynFlags`. We then call the `interactiveUI` function
 with args thusly: `interactiveUI defaultGhciSettings srcs maybe_expr`.
 This function comes from the `GHCi.UI` module in `ghc/GHCi/UI.hs`.

 So, we want to investigate `interactiveUI` in `ghc/GHCi/UI.hs`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15617#comment:17>
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