"Can't splice the polymorphic local variable" check looks dead
While I was looking for TH checks which might want to be checked when
Quotes are enabled ala #10382 (closed), I found the "Can't splice the polymorphic local variable" check in
TcExpr. I thought this was a bit odd, because there is another implementation of this very function in
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) [<xs, lift xs>, <z, lift z>, <f, lift f>]’ In the expression: runQ [| foldr f z xs |] pending(rn) [<xs, lift xs>, <z, lift z>, <f, lift f>]
which seems to be induced the check in
So is it dead? If so, let's remove it!