Fix type-on-hover over non-dependent function types (#1233)

Previously, hovering over any part of a non-dependent function type like
`Bool -> Bool` would result in the type of the domain being displayed
(`Bool` in this example). The correct type is `Type`.

This bug resulted from the fact that we
expected all functions types to be syntactically of the form
`forall (b : Bool) -> Bool`, disregarding the simpler syntax for the
non-dependent function space (which desugars to
`forall (_ : Bool) -> Bool`).

dhall-lsp-server/src/Dhall/LSP/Backend/Parsing.hs

@@ -81,11 +81,9 @@ getLetIdentifier src@(Src left _ text) =
           return (Src begin end tokens)
 
 -- | Cf. `getLetIdentifier`.
-getLamIdentifier :: Src -> Src
-getLamIdentifier src@(Src left _ text) =
-  case Megaparsec.parseMaybe (unParser parseLetIdentifier) text of
-    Just src' -> src'
-    Nothing -> src
+getLamIdentifier :: Src -> Maybe Src
+getLamIdentifier (Src left _ text) =
+  Megaparsec.parseMaybe (unParser parseLetIdentifier) text
   where parseLetIdentifier = do
           setSourcePos left
           _lambda
@@ -97,11 +95,9 @@ getLamIdentifier src@(Src left _ text) =
           return (Src begin end tokens)
 
 -- | Cf. `getLetIdentifier`.
-getForallIdentifier :: Src -> Src
-getForallIdentifier src@(Src left _ text) =
-  case Megaparsec.parseMaybe (unParser parseLetIdentifier) text of
-    Just src' -> src'
-    Nothing -> src
+getForallIdentifier :: Src -> Maybe Src
+getForallIdentifier (Src left _ text) =
+  Megaparsec.parseMaybe (unParser parseLetIdentifier) text
   where parseLetIdentifier = do
           setSourcePos left
           _forall

dhall-lsp-server/src/Dhall/LSP/Backend/Typing.hs

@@ -39,12 +39,14 @@ typeAt' pos ctx (Note src (Let (Binding _ _ a :| []) _)) | pos `inside` getLetId
   return (Just $ getLetIdentifier src, typ)
 
 -- "..." in a lambda expression
-typeAt' pos _ctx (Note src (Lam _ _A _)) | pos `inside` getLamIdentifier src =
-  return (Just $ getLamIdentifier src, _A)
+typeAt' pos _ctx (Note src (Lam _ _A _)) | Just src' <- getLamIdentifier src
+                                         , pos `inside` src' =
+  return (Just src', _A)
 
 -- "..." in a forall expression
-typeAt' pos _ctx (Note src (Pi _ _A _)) | pos `inside` getForallIdentifier src =
-  return (Just $ getForallIdentifier src, _A)
+typeAt' pos _ctx (Note src (Pi _ _A _)) | Just src' <- getForallIdentifier src
+                                        , pos `inside` src' =
+  return (Just src', _A)
 
 -- the input only contains singleton lets
 typeAt' pos ctx (Let (Binding x _ a :| []) e@(Note src _)) | pos `inside` src = do