[GHC] #13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification

[GHC]

#13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and
ExistentialQuantification
-------------------------------------+-------------------------------------
        Reporter:  ljli              |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.1
      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 simonpj):

 OK here's what I can do easily.  Let's say that in a `Stmt`
 {{{
   pat <- rhs
 }}}

 * the `Stmt` ''requires'' constraints needed to typecheck `rhs`, and to
 pattern-match `pat` (e.g. in a view pattern)
 * the `Stmt` ''binds'' the existential type variables and constraints
 brought into scope by the pattern `pat`

 So for example, given
 {{{
 data T a where
   MkT :: (Eq a, Show b) => a -> b -> T a
 }}}
 Suppose that `v :: t` is in scope. Then the stmt
 {{{
   MkT 1 x <- (...show v...) :: T t
 }}}
 * requires `Show t` (from the use of `show`) and `Num t` (from the literal
 `1`).
 * binds the existential `b` and constraint `Show b`.

 Now consider
 {{{
    ...stmts...
    ApplicativeStmts [arg1, arg2, ... argN]
    ...more stmts...
 }}}
 where `argi :: ApplicativeArg`. Each `argi` itself contains one or more
 `Stmts`.
 It is easy to ensure that

 * Constraints required by the `argi` can be solved from constraint bound
 by `...stmts...`
 * Constraints and existentials bound by the `argi` are not available to
 solve constraints required either by `argj` (where i is different from j),
 or by `...more stmts...`.
 * Within the stmts of each `argi` individually, constraints bound by
 earlier stmts can be used to solve later ones.

 That is easy to implement and solves the immediate problem.  I'm
 validating now.

 ------------------------
 But the rule is terribly unsatisfactory.

 * To typecheck the program you must mentally desugar it into its
 applicative groups.

 * If you write
 {{{
 MkT x1 _ <- rhs1
 MkT x2 _ <- rhs2
 }}}
   you really might intend that the `Eq t` bound by the first stmt is
 available to solve requirements of `rhs2`.  And with normal monadic
 deguaring it would be, but not with `ApplicativeDo`.  Worse, ''there is no
 way to fix it'' becuase `ApplicativeDo` works module-wide.

 My solution is: add syntax to allow the programmer to express the program
 in the form of Figure 2 of our paper, that is, post-rearrangement.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13242#comment:8>
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