Typed template haskell quote fails to typecheck when spliced due to an ambiguous type variable
It should be the case that a code value constructed using typed template haskell should never fail to type check when spliced. Running ghc Test.hs
with the following two modules produces an error about an ambiguous type variable.
https://gist.github.com/5890c14dda73da738d2041c7f677b786
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -Wall #-}
module Compiler where
import Language.Haskell.TH
data Operator = Scan
| Join Operator Operator deriving Show
queryJoin :: Operator
queryJoin = Join Scan Scan
type QTExp a = Q (TExp a)
fix :: (a -> a) -> a
fix f = let x = f x in x
while ::
Monoid m =>
QTExp (IO m -> IO m) -> QTExp (IO m)
while b = [|| fix (\r -> whenM True ($$b r)) ||]
whenM :: Monoid m => Bool -> m -> m
whenM b act = if b then act else mempty
execOp :: Monoid m => Operator -> QTExp (IO m) -> QTExp (IO m)
execOp op yld =
case op of
Scan ->
while [|| \r -> ($$(yld) >> r)||]
Join left right ->
execOp left (execOp right yld)
runQuery :: QTExp (IO ())
runQuery = execOp (Join Scan Scan) ([|| return () ||])
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Test where
import qualified Compiler as C
main :: IO ()
main = do
$$(C.runQuery)
Test.hs:9:6: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘C.whenM’
prevents the constraint ‘(Monoid a0)’ from being solved.
Relevant bindings include r_a5GX :: IO a0 (bound at Test.hs:9:6)
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Monoid a => Monoid (IO a) -- Defined in ‘GHC.Base’
instance Monoid Ordering -- Defined in ‘GHC.Base’
instance Semigroup a => Monoid (Maybe a) -- Defined in ‘GHC.Base’
...plus 7 others
(use -fprint-potential-instances to see them all)
• In the expression:
(C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX)
In the first argument of ‘C.fix’, namely
‘(\ r_a5GX
-> (C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX))’
In the first argument of ‘(>>)’, namely
‘(C.fix
(\ r_a5GX
-> (C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX)))’
|
9 | $$(C.runQuery)
|
The generated code
Test.hs:9:6-15: Splicing expression
C.runQuery
======>
C.fix
(\ r_a5GV
-> (C.whenM True)
((\ r_a5GW
-> ((C.fix
(\ r_a5GX
-> (C.whenM True)
((\ r_a5GY -> ((return GHC.Tuple.()) >> r_a5GY)) r_a5GX)))
>> r_a5GW))
r_a5GV))
Trac metadata
Trac field | Value |
---|---|
Version | 8.6.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |