[GHC] #11708: Typechecker hangs when checking type families with -ddump-tc-trace turned on
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ghc-devs at haskell.org
Tue Mar 15 00:57:02 UTC 2016
#11708: Typechecker hangs when checking type families with -ddump-tc-trace turned
on
-------------------------------------+-------------------------------------
Reporter: kcsongor | Owner: kcsongor
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.1
(Type checker) |
Keywords: typefamilies | Operating System: Unknown/Multiple
trace hangs |
Architecture: | Type of failure: None/Unknown
Unknown/Multiple |
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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I have the following code, which is supposed to fail with a type error.
However, when compiled with HEAD using the -ddump-tc-trace flag, the type
checker hangs.
{{{#!hs
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DataKinds #-}
module Test where
import Data.Proxy
test :: (IsElem a as ~ 'True) => Proxy a -> Proxy as -> Bool
test _ _ = True
x = test (Proxy :: Proxy Int) (Proxy :: Proxy '[])
type family IsElem (x :: k) (xs :: [k]) where
IsElem x '[] = 'False
IsElem x (x ': xs) = 'True
IsElem x (y ': xs) = IsElem x xs
}}}
The reason is that in typecheck/TcHsType.hs, the type family tycons are
type-checked with knot-tying, however, they are being traced, forcing
their evaluation which causes the typechecker to hang.
My proposed fix is to only print the safe values that we know are
constructed by the time of tracing.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11708>
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