[GHC] #13530: Horrible error message due to TypeInType
GHC
ghc-devs at haskell.org
Wed Apr 5 12:48:24 UTC 2017
#13530: Horrible error message due to TypeInType
-------------------------------------+-------------------------------------
Reporter: simonpj | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.0.1
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: None/Unknown
Unknown/Multiple |
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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Consider this
{{{
{-# LANGUAGE MagicHash, UnboxedTuples #-}
module Foo where
import GHC.Exts
g :: Int -> (# Int#, a #)
g (I# y) = (# y, undefined #)
f :: Int -> (# Int#, Int# #)
f x = g x
}}}
With GHC 8 we get
{{{
Foo.hs:11:7: error:
• Couldn't match a lifted type with an unlifted type
Expected type: (# Int#, Int# #)
Actual type: (# Int#, Int# #)
}}}
What a terrible error message!! It was much better in GHC 7.10:
{{{
Foo.hs:11:7:
Couldn't match kind ‘*’ with ‘#’
When matching types
a0 :: *
Int# :: #
Expected type: (# Int#, Int# #)
Actual type: (# Int#, a0 #)
}}}
What's going on?
The constraint solver sees
{{{
[W] alpha::TYPE LiftedRep ~ Int#::TYPE IntRep
}}}
So it homogenises the kinds, ''and unifies alpha'' (this did not happen in
GHC 7.10), thus
{{{
alpha := Int# |> TYPE co
[W] co :: LiftedRep ~ IntRep
}}}
Of course the new constraint fails. But since we have unified alpha, when
we print out the types are are unifying they both look like `(# Int#, Int#
#)` (there's a suppressed cast in the second component).
I'm not sure what to do here.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13530>
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