Allow template-haskell 2.17, 2.18 (#34)

With a bit of CPP for backwards compatibility.

Ref https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#template-haskell-217

package.yaml

@@ -57,7 +57,7 @@ library:
   dependencies:
   - natural-transformation >= 0.2
   - transformers-base
-  - template-haskell >= 2.11 && < 2.17
+  - template-haskell >= 2.11 && < 2.19
 
 executables:
   freer-examples:

src/Control/Monad/Freer/TH.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TemplateHaskell #-}
 
@@ -121,7 +122,11 @@ genType (GadtC   _ tArgs' (AppT eff tRet)) = do
     resultType       = ConT ''Eff `AppT` VarT effs `AppT` tRet
 
   return
+#if MIN_VERSION_template_haskell(2,17,0)
+    .  ForallT [PlainTV effs SpecifiedSpec] [memberConstraint]
+#else
     .  ForallT [PlainTV effs] [memberConstraint]
+#endif
     .  foldArrows
     $  tArgs
     ++ [resultType]
@@ -141,8 +146,13 @@ simplifyBndrs t = t
 
 -- | Turn TvVarBndrs of the form (KindedTV tv StarT) into (PlainTV tv)
 -- This can prevent the need for KindSignatures
+#if MIN_VERSION_template_haskell(2,17,0)
+simplifyBndr :: TyVarBndrSpec -> TyVarBndrSpec
+simplifyBndr (KindedTV tv f StarT) = PlainTV tv f
+#else
 simplifyBndr :: TyVarBndr -> TyVarBndr
 simplifyBndr (KindedTV tv StarT) = PlainTV tv
+#endif
 simplifyBndr bndr = bndr
 
 -- | Generates a type signature of the form