Extract definition of DsM into GHC.HsToCore.Types

`DsM` was previously defined in `GHC.Tc.Types`, along with `TcM`. But
`GHC.Tc.Types` is in the set of transitive dependencies of `GHC.Parser`,
a set which we aim to minimise. Test case `CountParserDeps` checks for
that.

Having `DsM` in that set means the parser also depends on the innards of
the pattern-match checker in `GHC.HsToCore.PmCheck.Types`, which is the
reason we have that module in the first place.

In the previous commit, we represented the `TyState` by an `InertSet`,
but that pulls the constraint solver as well as 250 more modules into
the set of dependencies, triggering failure of `CountParserDeps`.
Clearly, we want to evolve the pattern-match checker (and the desugarer)
without being concerned by this test, so this patch includes a small
refactor that puts `DsM` into its own module.

1 files changed, 8 insertions, 15 deletions

diff --git a/compiler/GHC/Tc/Utils/Monad.hs b/compiler/GHC/Tc/Utils/Monad.hs
index b2c987794b..96bff3d261 100644
--- a/compiler/GHC/Tc/Utils/Monad.hs
+++ b/compiler/GHC/Tc/Utils/Monad.hs
@@ -138,7 +138,7 @@ module GHC.Tc.Utils.Monad(
   withException,
 
   -- * Stuff for cost centres.
-  ContainsCostCentreState(..), getCCIndexM,
+  getCCIndexM, getCCIndexTcM,
 
   -- * Types etc.
   module GHC.Tc.Types,
@@ -2081,23 +2081,16 @@ discussion).  We don't currently know a general solution to this problem, but
 we can use uninterruptibleMask_ to avoid the situation.
 -}
 
--- | Environments which track 'CostCentreState'
-class ContainsCostCentreState e where
-  extractCostCentreState :: e -> TcRef CostCentreState
-
-instance ContainsCostCentreState TcGblEnv where
-  extractCostCentreState = tcg_cc_st
-
-instance ContainsCostCentreState DsGblEnv where
-  extractCostCentreState = ds_cc_st
-
 -- | Get the next cost centre index associated with a given name.
-getCCIndexM :: (ContainsCostCentreState gbl)
-            => FastString -> TcRnIf gbl lcl CostCentreIndex
-getCCIndexM nm = do
+getCCIndexM :: (gbl -> TcRef CostCentreState) -> FastString -> TcRnIf gbl lcl CostCentreIndex
+getCCIndexM get_ccs nm = do
   env <- getGblEnv
-  let cc_st_ref = extractCostCentreState env
+  let cc_st_ref = get_ccs env
   cc_st <- readTcRef cc_st_ref
   let (idx, cc_st') = getCCIndex nm cc_st
   writeTcRef cc_st_ref cc_st'
   return idx
+
+-- | See 'getCCIndexM'.
+getCCIndexTcM :: FastString -> TcM CostCentreIndex
+getCCIndexTcM = getCCIndexM tcg_cc_st