#2102: Typeclass membership doesn't bring coercion superclass requirements into
scope
----------------------------------------+-----------------------------------
Reporter: ryani | Owner: chak
Type: bug | Status: new
Priority: low | Milestone: 7.0.1
Component: Compiler (Type checker) | Version: 7.1
Keywords: superclass equalities | Testcase:
Blockedby: | Difficulty: Unknown
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Compile-time crash
----------------------------------------+-----------------------------------
Changes (by reinerp):
* failure: => Compile-time crash
* version: 6.8.2 => 7.1
Comment:
With 7.0.0-rc1 ({{{7.0.0.20100924}}}) and in HEAD, the original testcase
reported now compiles successfully (and passes core lint as well).
The motivating example (with coerce) in Ryan's comment also compiles, but
fails core lint:
{{{
*** Core Lint errors : in result of Desugar ***
<no location info>:
In the coercion `co_af4'
co_af4 is out of scope
*** Offending Program ***
T2102.$fValid() [InlPrag=[ALWAYS] CONLIKE] :: T2102.Valid ()
[LclIdX[DFunId], Unf=DFun(arity=0) T2102.D:Valid []]
T2102.$fValid() =
T2102.D:Valid @ () @ (trans (T2102.TFCo:R:Cat()ts' ()) ())
T2102.$fValid(,) [InlPrag=[ALWAYS] CONLIKE]
:: forall ts_aaM s_aaN.
T2102.Valid ts_aaM =>
T2102.Valid (s_aaN, ts_aaM)
[LclIdX[DFunId], Unf=DFun(arity=3) T2102.D:Valid []]
T2102.$fValid(,) =
\ (@ ts_aaM) (@ s_aaN) _ ->
T2102.D:Valid @ (s_aaN, ts_aaM) @ co_af4
T2102.coerce
:: forall ts_aaJ (f_aaK :: * -> *).
T2102.Valid ts_aaJ =>
f_aaK (T2102.Cat ts_aaJ ()) -> f_aaK ts_aaJ
[LclIdX]
T2102.coerce =
\ (@ ts_aeV)
(@ f_aeW::* -> *)
($dValid_aeX :: T2102.Valid ts_aeV) ->
case $dValid_aeX of _ { T2102.D:Valid @ co_afa ->
\ (x_aaL :: f_aeW (T2102.Cat ts_aeV ())) ->
x_aaL
`cast` (f_aeW (trans co_afa ts_aeV)
:: f_aeW (T2102.Cat ts_aeV ()) ~ f_aeW ts_aeV)
}
*** End of Offense ***
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2102#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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