Comments and tweaks to SMethodMap

Author: michaelpj

lsp-types/src/Language/LSP/Types/SMethodMap.hs

@@ -21,13 +21,22 @@ import Data.Kind (Type)
 import Data.Map (Map)
 import qualified Data.Map.Strict as Map
 import Data.Text (Text)
-import GHC.Exts (Int(..), dataToTag#)
+import GHC.Exts (Int(..), dataToTag#, Any)
 import Unsafe.Coerce (unsafeCoerce)
 
 import Language.LSP.Types.Method (Method(..), SMethod(..))
 
+-- This type exists to avoid a dependency on 'dependent-map'. It is less
+-- safe (since we use 'unsafeCoerce') but much simpler and hence easier to include.
+-- | A specialized altenative to a full dependent map for use with 'SMethod'.
 data SMethodMap (v :: Method f t -> Type) =
-  SMethodMap !(IntMap (v 'CustomMethod)) !(Map Text (v 'CustomMethod))
+  -- This works by using an 'IntMap' indexed by constructor tag for the majority
+  -- of 'SMethod's, which have no parameters, and hence can only appear once as keys
+  -- in the map. We do not attempt to be truly dependent here, and instead exploit
+  -- 'usafeCoerce' to go to and from 'v Any'.
+  -- The sole exception is 'SCustomMethod', for which we keep a separate map from
+  -- its 'Text' parameter (and where we can get the type indices right).
+  SMethodMap !(IntMap (v Any)) !(Map Text (v 'CustomMethod))
 
 toIx :: SMethod a -> Int
 toIx k = I# (dataToTag# k)