diff --git a/src/Misc.hs b/src/Misc.hs
index 160928f..633eb14 100644
--- a/src/Misc.hs
+++ b/src/Misc.hs
@@ -12,10 +12,14 @@ import Prelude hiding (map)
 lookupDefault :: Ord k => a -> k -> Map k a -> a
 lookupDefault d k m = fromMaybe d (M.lookup k m)
 
-infixl \\
+infixl 3 \\
 (\\) :: Ord a => Set a -> Set a -> Set a
 (\\) = S.difference
 
+infixl 3 \-
+(\-) :: Ord a => Set a -> a -> Set a
+(\-) = flip S.delete
+
 infix 5 <~>
 (<~>) :: (Traversable t, Applicative f) => (a -> f b) -> t a -> f (t b)
 (<~>) = traverse
diff --git a/src/Toplevel.hs b/src/Toplevel.hs
index 23e2c33..981d5ce 100644
--- a/src/Toplevel.hs
+++ b/src/Toplevel.hs
@@ -1,27 +1,35 @@
+{-# LANGUAGE LambdaCase, TupleSections #-}
 module Toplevel (check) where
 
 import Control.Monad.Except hiding (guard)
 import Control.Monad.RWS    hiding (guard)
 
 import Data.Map (Map)
+import Data.Set (Set)
 import qualified Data.Map as M
 import qualified Data.Set as S
 
+import Data.Maybe (mapMaybe)
+
 import TC
 import TC.Helpers
 import Solve
 import Type
-
+import Misc
 import PostProcess
 
 import qualified Hm.Abs as H
 
+import Prelude hiding (map)
+
 check :: [H.Def] -> Either TypeError [TL]
 check defs = case runInfer M.empty (traverseInfer defs) of
   Left err -> throwError err
   Right (tls,_,cs) -> case runSolve (emptySubst, cs) of
     Left err -> throwError err
-    Right sub -> pure tls
+    Right sub -> case runExcept (detectMutualRecursion tls) of
+      Left err -> throwError err
+      Right () -> pure tls
 
 
 traverseInfer :: [H.Def] -> Infer [TL]
@@ -61,3 +69,37 @@ accumulateEnv (t:ts) = case t of
 
 
   _ -> throwError Oop
+
+detectMutualRecursion :: [TL] -> Except TypeError ()
+detectMutualRecursion = detectLoops . createGraph
+
+detectLoops :: RefGraph -> Except TypeError ()
+detectLoops g = case M.minViewWithKey g of -- idk a better way to just get a key
+  Nothing    -> pure ()
+  Just ((i,_),_) -> detectLoop S.empty g i >>= detectLoops . foldr M.delete g -- remove all visited nodes
+
+detectLoop :: Set Id -> RefGraph -> Id -> Except TypeError (Set Id)
+detectLoop v g i =
+  if S.member i v
+     then throwError (MutuallyRecursive v) -- already visited, loop
+     else case M.lookup i g of
+       Nothing -> pure v -- this has already been removed, meaning this branch is safe
+       Just is -> if is == S.empty
+                     then pure v -- leaf                               \/ Set not Traversable
+                     else S.unions <$> detectLoop (S.insert i v) g <~> S.toList is
+
+createGraph :: [TL] -> RefGraph
+createGraph = M.fromList . mapMaybe createNode
+
+createNode :: TL -> Maybe RefNode
+createNode TypeDef{} = Nothing
+createNode (VarDef _ i _ e) = Just (i, refs e)
+  where
+    refs :: Exp -> Set Id
+    refs = \case
+      Let _ []  e2          -> refs e2
+      Let p ((i,e1):ies) e2 -> refs e1 <> refs (Let p ies e2) \- i
+
+      Abs _ is e2  -> refs e2 \\ S.fromList is
+      App _ e1 e2s -> refs e1 <> foldMap refs e2s
+      Var _ i      -> S.singleton i
diff --git a/src/Type.hs b/src/Type.hs
index 117d011..129511d 100644
--- a/src/Type.hs
+++ b/src/Type.hs
@@ -124,6 +124,7 @@ data TypeError
   | InvalidConstructor
   | ArityMismatch
   | AlreadyDefined Pos Id
+  | MutuallyRecursive (Set Id)
   deriving Show
 
 type TypeEnv = Map Id PolyT
@@ -173,3 +174,6 @@ newtype Solve a = Solve { getSolve :: ReaderT Unifier (Except TypeError) a}
 
 instance MonadFail Solve where
   fail _ = throwError Oop
+
+type RefNode  = (Id, Set Id)
+type RefGraph = Map Id (Set Id)
diff --git a/test.hm b/test.hm
index 4c1aff2..f44dd8a 100644
--- a/test.hm
+++ b/test.hm
@@ -51,3 +51,7 @@ eval : Expr → Nat
 -- here's some other functions
 isEven : Nat → Bool
  := rec[Nat] true not
+
+type bot
+
+absurd : bot -> A := rec[bot]