diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -2,6 +2,15 @@
 
 `dead-code-detection` detects dead code in haskell projects.
 
+This project is still in an early stage. Currently only those language
+constructs have been implemented that I have stumbled across in projects I used
+it on. If the tool encounters a language construct that it doesn't understand
+yet, it will crash. (I *think* this is the best behavior since ignoring
+unimplemented language constructs would easily result in false positives and
+false negatives.) If you use `dead-code-detection` on any project and it
+doesn't work due to a not implemented language construct, please consider
+opening an issue on github.
+
 ``` shell
 $ dead-code-detection --root Main -isrc
 src/Example/Module.hs:42:23: unusedName
diff --git a/dead-code-detection.cabal b/dead-code-detection.cabal
--- a/dead-code-detection.cabal
+++ b/dead-code-detection.cabal
@@ -3,7 +3,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           dead-code-detection
-version:        0.5
+version:        0.5.1
 synopsis:       detect dead code in haskell projects
 description:    detect dead code in haskell projects
 category:       Development
@@ -75,6 +75,7 @@
     , filepath
     , directory
   other-modules:
+      Ast.UsedNamesSpec
       AstSpec
       FilesSpec
       GraphSpec
diff --git a/src/Ast.hs b/src/Ast.hs
--- a/src/Ast.hs
+++ b/src/Ast.hs
@@ -27,7 +27,6 @@
 import           System.IO.Silently
 
 import           Ast.UsedNames
-import           GHC.Show
 import           Graph
 
 type Ast = [Module]
@@ -141,12 +140,12 @@
       nameGraph valBinds ++
       nameGraph tyclds ++
       nameGraph foreign_decls
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance NameGraph (ForeignDecl Name) where
   nameGraph = \ case
     ForeignImport name _ _ _ -> [(unLoc name, [])]
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance NameGraph (HsValBinds Name) where
   nameGraph = \ case
@@ -157,7 +156,7 @@
   nameGraph bind = addUsedNames (usedNames bind) $ case bind of
     FunBind id _ _ _ _ _ -> withoutUsedNames [unLoc id]
     PatBind pat _ _ _ _ -> nameGraph pat
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance NameGraph (Pat Name) where
   nameGraph = \ case
@@ -166,19 +165,19 @@
     VarPat p -> withoutUsedNames [p]
     TuplePat pats _ _ -> nameGraph pats
     WildPat _ -> []
-    pat -> nyi "Pat" pat
+    pat -> errorNyiOutputable pat
 
 instance NameGraph (TyClGroup Name) where
   nameGraph = \ case
     TyClGroup decls [] -> nameGraph decls
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance NameGraph (TyClDecl Name) where
   nameGraph = \ case
     DataDecl _typeCon _ def _ -> nameGraph def
     ClassDecl{} -> []
     SynDecl{} -> []
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance NameGraph (HsDataDefn Name) where
   nameGraph = \ case
@@ -194,7 +193,7 @@
   nameGraph = \ case
     RecCon rec -> nameGraph rec
     PrefixCon _ -> []
-    x -> e x
+    x -> errorNyiData x
 
 instance NameGraph (ConDeclField Name) where
   nameGraph = \ case
@@ -205,7 +204,7 @@
   nameGraph = \ case
     PrefixCon args -> nameGraph args
     InfixCon a b -> nameGraph a ++ nameGraph b
-    _ -> error "Not yet implemented: HsConPatDetails"
+    x -> errorNyiData x
 
 -- | extracts names used in instance declarations
 getClassMethodUsedNames :: Ast -> [Name]
diff --git a/src/Ast/UsedNames.hs b/src/Ast/UsedNames.hs
--- a/src/Ast/UsedNames.hs
+++ b/src/Ast/UsedNames.hs
@@ -8,6 +8,7 @@
 import           Data.Data
 import           GHC
 import           Outputable
+import           Utils
 
 class UsedNames ast where
   -- | extracts all used names from ASTs
@@ -27,7 +28,7 @@
     FunBind _ _ matches _ _ _ -> usedNames matches
     PatBind lhs rhs _ _ _ ->
       usedNames lhs ++ usedNames rhs
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (MatchGroup Name (LHsExpr Name)) where
   usedNames = usedNames . mg_alts
@@ -52,7 +53,7 @@
     BangPat pat -> usedNames pat
     NPat{} -> []
     LitPat{} -> []
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (HsConDetails (LPat Name) (HsRecFields Name (LPat Name))) where
   usedNames = \ case
@@ -76,7 +77,7 @@
     LastStmt expr _ -> usedNames expr
     BodyStmt expr _ _ _ -> usedNames expr
     LetStmt x -> usedNames x
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (HsExpr Name) where
   usedNames = \ case
@@ -110,7 +111,7 @@
     ArithSeq _ _ info -> usedNames info
     RecordCon constructor _ binds ->
       unLoc constructor : usedNames binds
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (ArithSeqInfo Name) where
   usedNames = \ case
@@ -129,19 +130,19 @@
     MDoExpr -> []
     ArrowExpr -> []
     GhciStmtCtxt -> []
-    x -> e x
+    x -> errorNyiData x
 
 instance UsedNames (HsLocalBinds Name) where
   usedNames = \ case
     EmptyLocalBinds -> []
     HsValBinds binds -> usedNames binds
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (HsValBindsLR Name Name) where
   usedNames = \ case
     ValBindsOut (map snd -> binds) _sig ->
       usedNames binds
-    x -> o x
+    x -> errorNyiOutputable x
 
 instance UsedNames (HsTupArg Name) where
   usedNames = \ case
@@ -156,14 +157,14 @@
   usedNames (HsRecField assigned expr _) =
     unLoc assigned : usedNames expr
 
-e :: (Data a) => a -> b
-e x = error $ ("e: " ++ ) $ unlines $
+errorNyiData :: (Data a) => a -> b
+errorNyiData x = errorNyi $ ("errorNyiData: " ++ ) $ unlines $
   dataTypeName (dataTypeOf x) :
   show (toConstr x) :
   []
 
-o :: (Outputable a, Data a) => a -> b
-o x = error $ ("o: " ++) $ unlines $
+errorNyiOutputable :: (Outputable a, Data a) => a -> b
+errorNyiOutputable x = errorNyi $ ("errorNyiOutputable: " ++) $ unlines $
   dataTypeName (dataTypeOf x) :
   show (toConstr x) :
   showSDocUnsafe (ppr x) :
diff --git a/src/GHC/Show.hs b/src/GHC/Show.hs
--- a/src/GHC/Show.hs
+++ b/src/GHC/Show.hs
@@ -18,9 +18,3 @@
         show (srcLocLine loc) ++ ":" ++
         show (srcLocCol loc)
       UnhelpfulLoc s -> cs (fs_bs s)
-
--- * development utils
-
-nyi :: Outputable doc => String -> doc -> a
-nyi msg x = error $
-  ("Not yet implemented: " ++ msg ++ " " ++ (showSDocUnsafe $ ppr x))
diff --git a/src/Utils.hs b/src/Utils.hs
--- a/src/Utils.hs
+++ b/src/Utils.hs
@@ -12,3 +12,13 @@
       | a `member` acc = inner acc r
       | otherwise = a : inner (insert a acc) r
     inner _ [] = []
+
+errorNyi :: String -> a
+errorNyi message = error $ unlines $
+  "Encountered a language construct that is" :
+  "not yet implemented. Please consider opening a bug report about" :
+  "this here: https://github.com/soenkehahn/dead-code-detection/issues" :
+  "" :
+  "Here's some debugging output that will probably help to solve this problem:" :
+  message :
+  []
diff --git a/test/Ast/UsedNamesSpec.hs b/test/Ast/UsedNamesSpec.hs
new file mode 100644
--- /dev/null
+++ b/test/Ast/UsedNamesSpec.hs
@@ -0,0 +1,26 @@
+
+module Ast.UsedNamesSpec where
+
+import           Control.Exception
+import           Data.Foldable
+import           Data.List
+import           Test.Hspec
+
+import           Ast.UsedNames
+
+spec :: Spec
+spec = do
+  let errs =
+        ("errorNyiData", errorNyiData ()) :
+        ("errorNyiOutputable", errorNyiOutputable ()) :
+        []
+  forM_ errs $ \ (name, err) -> do
+    describe name $ do
+      it "explains that this is not yet implemented" $ do
+        err () `shouldThrow` \ (ErrorCall message) ->
+          "not yet implemented" `isInfixOf` message
+
+      it "points to the issue tracker" $ do
+        err () `shouldThrow` \ (ErrorCall message) ->
+          "https://github.com/soenkehahn/dead-code-detection/issues" `isInfixOf`
+            message