2 files changed, 18 insertions, 15 deletions

diff --git a/src/Error/Base.hs b/src/Error/Base.hs
index d7b685c..ef32d52 100644
--- a/src/Error/Base.hs
+++ b/src/Error/Base.hs
@@ -1,8 +1,7 @@
-module Error.Base (
-             LispError (..)
-             , LispResult (..)
-             , unwrap
-             ) where
+module Error.Base ( LispError (..)
+                  , LispResult (..)
+                  , unwrap
+                  ) where
 
 import           Control.Monad.Except
 import           Parser
@@ -15,19 +14,24 @@ data LispError = Parse ParseError
                | ArgCount String Int [Expr]
                | UnknownFunction String
                | TypeMismatch String Expr
+               | UnboundVariable String
 
 unwordsList :: [Expr] -> String
 unwordsList = unwords . map show
 
+literal :: String -> String
+literal v = "`" <> v <> "`"
+
 instance Show LispError where
     show (Parse e)              = "Parser Error: " ++ show e
-    show (BadForm s expr)       = "Bad Form: " ++ s ++ ": " ++ show expr
+    show (BadForm s expr)       = "Bad Form: " ++ literal s ++ ": " ++ show expr
     -- TODO: clean this up
     show (ArgCount fn n es)
-      | null es = "Invalid arity, `" ++ fn ++ "` expects " ++ show n ++ " or more expression(s)!"
-      | otherwise = "Invalid arity, `" ++ fn ++ "` expects " ++ show n ++ " or more expression(s), got value(s): " ++ unwordsList es
-    show (UnknownFunction fn)   = "Cannot apply function: " ++ fn
-    show (TypeMismatch msg got) = "Type mismatch, expected " ++ msg ++ ", got: " ++ show got
+      | null es = "Invalid arity, " ++ literal fn ++ " expects " ++ show n ++ " or more expression(s)!"
+      | otherwise = "Invalid arity, " ++ literal fn ++ " expects " ++ show n ++ " or more expression(s), got value(s): " ++ unwordsList es
+    show (UnknownFunction fn)   = "Cannot apply function: " ++ literal fn
+    show (TypeMismatch msg got) = "Type mismatch, expected " ++ literal msg ++ ", got: " ++ show got
+    show (UnboundVariable name) = "Possibly unbound variable: " ++ literal name
 
 type LispResult = Either LispError
 
diff --git a/src/Error/Pretty.hs b/src/Error/Pretty.hs
index 44601d6..a90131b 100644
--- a/src/Error/Pretty.hs
+++ b/src/Error/Pretty.hs
@@ -1,8 +1,7 @@
-module Error.Pretty (
-     showError
-     , Options (..)
-     , defaults
-    ) where
+module Error.Pretty ( showError
+                    , Options (..)
+                    , defaults
+                    ) where
 
 import           Data.List                     (intercalate, nub)
 import           Error.Base                    (LispError (..))