#10384: "Can't splice the polymorphic local variable" check looks dead
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Reporter: ezyang | Owner:
Type: task | Status: new
Priority: normal | Milestone:
Component: Template | Version: 7.11
Haskell | Operating System: Unknown/Multiple
Keywords: | Type of failure: None/Unknown
Architecture: | Blocked By:
Unknown/Multiple | Related Tickets:
Test Case: |
Blocking: |
Differential Revisions: |
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While I was looking for TH checks which might want to be checked when
`Quotes` are enabled ala #10382, I found the "Can't splice the polymorphic
local variable" check in `checkCrossStageLifting` in `TcExpr`. I thought
this was a bit odd, because there is another implementation of this very
function in `RnSplice`.
So I went ahead and removed this check from the compiler, and it validated
fine.
I then went and tried to see if I could tickle the problem. An old mailing
list suggested the following test program to induce the error:
{{{
module TH_polymorphic where
import Language.Haskell.TH
import Language.Haskell.TH.Syntax
-- See https://mail.haskell.org/pipermail/template-
haskell/2006-April/000552.html
test2 () = runQ [| foldr f z xs |]
where (f,z,xs) = undefined
}}}
but I get this error:
{{{
TH_polymorphic.hs:8:17: error:
Could not deduce (Lift t0) arising from a use of ‘lift’
from the context: Quasi m
bound by the inferred type of test2 :: Quasi m => () -> m Exp
at TH_polymorphic.hs:(8,1)-(9,30)
The type variable ‘t0’ is ambiguous
Note: there are several potential instances:
instance (Lift a, Lift b) => Lift (Either a b)
-- Defined in ‘Language.Haskell.TH.Syntax’
instance Lift a => Lift (Maybe a)
-- Defined in ‘Language.Haskell.TH.Syntax’
instance Lift Int16 -- Defined in ‘Language.Haskell.TH.Syntax’
...plus 24 others
In the expression: lift xs
In the first argument of ‘runQ’, namely
‘[| foldr f z xs |]
pending(rn) [