#1651: panic when interactively evaluating expression with a family type
--------------------------------------+-------------------------------------
Reporter: chak | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.7
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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{{{
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE EmptyDataDecls #-}
newtype Succ a = Succ a deriving Show
data Zero = Zero
instance Show Zero where show _ = "Zero"
type family Add a b
type instance Add Zero a = a
type instance Add (Succ n) m = Succ (Add n m)
add :: a -> b -> Add a b
add = undefined
okay = show $ add Zero Zero
bad = add Zero Zero
{- ghci transcript:
*Main> okay
"Zero"
*Main> bad
ghc-6.7.20070828: panic! (the 'impossible' happened)
(GHC version 6.7.20070828 for i386-unknown-linux):
readFilledBox t_a1D9{tv} [box]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
*Main> show bad
<interactive>:1:0:
No instance for (Show (Add Zero Zero))
arising from a use of `show' at <interactive>:1:0-7
Possible fix:
add an instance declaration for (Show (Add Zero Zero))
In the expression: show bad
In the definition of `it': it = show bad
-}
}}}
The panic arises as follows: `tcGhciStmts` calls `TcMatches.tcDoStmt` to
type check `it <- bad`, which in turn evaluates
{{{
withBox liftedTypeKind $ \ pat_ty ->
tcMonoExpr rhs (mkAppTy m_ty pat_ty)
}}}
The `withBox` executes the `readfilledBox` that causes the panic, as
`tcMonoExpr` promises to fill the boxes in its second argument. This
promise is not fulfilled, as `tcMonoBox` defers the match `Add Zero Zero ~
IO t_ayP`.
Further up the call chain `tcGhciStmts` will eventually simplify the LIE
and would discover the type mismatch (and hence abandon "Plan A", of which
all this is part). Unfortunately, we never get there due to `withBox`s
attempt to read `t_avP`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1651>
GHC <http://www.haskell.org/ghc/>
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