[GHC] #8227: cgLookupPanic (probably invalid Core

[GHC]

#8227: cgLookupPanic (probably invalid Core
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        Reporter:  guest               |            Owner:
            Type:  bug                 |           Status:  closed
        Priority:  normal              |        Milestone:
       Component:  Compiler            |          Version:  7.6.3
      Resolution:  fixed               |         Keywords:  cgLookupPanic
Operating System:  MacOS X             |     Architecture:
 Type of failure:  Compile-time crash  |  Unknown/Multiple
       Test Case:                      |       Difficulty:  Unknown
        Blocking:                      |       Blocked By:
                                       |  Related Tickets:
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Changes (by monoidal):

 * status:  new => closed
 * resolution:   => fixed


Comment:

 I distilled the part that causes panic in 7.6.3 and that part gives a
 correct type error in HEAD.

 The crucial part is this:

 {{{
 arcLengthToParam :: Scalar (V p) -> p -> Scalar (V p) -> Scalar (V p)

 absoluteToParam :: Scalar (V a) -> a -> Scalar (V a) -> Scalar (V a)
 absoluteToParam eps seg len  = arcLengthToParam eps (arcLength eps seg -
 len)

 -- You probably wanted
 absoluteToParam eps seg len  = arcLengthToParam eps seg (arcLength eps seg
 - len)
 }}}

 By skipping this parameter GHC has to solve `Scalar (V a) ~ a` and `Scalar
 (V a) -> Scalar (V p) ~ Scalar (V p)` and gets a headache. Here's a
 selfcontained test that panicks 7.6.3 gives occurs check in HEAD:

 {{{
 {-# LANGUAGE TypeFamilies #-}
 module V where
 type family V a :: *

 type instance V Double    = Double
 type instance V (a -> b)   = V b

 {-# LANGUAGE TypeFamilies #-}
 module Parametric
   (
     absoluteToParam
   ) where

 import V

 type family Scalar a :: *
 type instance Scalar (a -> v) = a -> Scalar v

 arcLengthToParam :: Scalar (V p) -> p -> Scalar (V p) -> Scalar (V p)
 arcLengthToParam = undefined

 absoluteToParam :: Scalar (V a) -> a -> Scalar (V a)
 absoluteToParam eps seg = arcLengthToParam eps eps
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8227#comment:2>
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