diff --git a/llvm-tf.cabal b/llvm-tf.cabal
--- a/llvm-tf.cabal
+++ b/llvm-tf.cabal
@@ -1,5 +1,5 @@
 Name:          llvm-tf
-Version:       12.0
+Version:       12.0.0.1
 License:       BSD3
 License-File:  LICENSE
 Synopsis:      Bindings to the LLVM compiler toolkit using type families.
@@ -37,7 +37,7 @@
   Location: http://code.haskell.org/~thielema/llvm-tf/
 
 Source-Repository this
-  Tag:      12.0
+  Tag:      12.0.0.1
   Type:     darcs
   Location: http://code.haskell.org/~thielema/llvm-tf/
 
diff --git a/private/LLVM/Core/CodeGen.hs b/private/LLVM/Core/CodeGen.hs
--- a/private/LLVM/Core/CodeGen.hs
+++ b/private/LLVM/Core/CodeGen.hs
@@ -475,12 +475,12 @@
     String -> (String -> CodeGenModule FFI.ValueRef) ->
     CodeGenFunction r (Value ptr)
 externCore name act = do
-    es <- getExterns
-    case lookup name es of
+    mf <- lookupExtern name
+    case mf of
         Just f -> return $ Value f
         Nothing -> do
             f <- liftCodeGenModule $ act name
-            putExterns ((name, f) : es)
+            addExtern name f
             return $ Value f
 
 {- |
diff --git a/private/LLVM/Core/CodeGenMonad.hs b/private/LLVM/Core/CodeGenMonad.hs
--- a/private/LLVM/Core/CodeGenMonad.hs
+++ b/private/LLVM/Core/CodeGenMonad.hs
@@ -6,7 +6,8 @@
     GlobalMappings(..), addGlobalMapping, getGlobalMappings,
     addFunctionMapping,
     -- * Function code generation
-    CodeGenFunction, runCodeGenFunction, liftCodeGenModule, genFSym, getFunction, getBuilder, getFunctionModule, getExterns, putExterns,
+    CodeGenFunction, runCodeGenFunction, liftCodeGenModule, genFSym,
+    getFunction, getBuilder, getFunctionModule, lookupExtern, addExtern,
     ) where
 
 import qualified LLVM.Core.Data as Data
@@ -23,6 +24,9 @@
 import Control.Monad.IO.Class (MonadIO, liftIO, )
 import Control.Monad (when, )
 import Control.Applicative (Applicative, )
+
+import qualified Data.Map as Map
+import Data.Map (Map)
 import Data.Monoid (Monoid, mempty, mappend, )
 import Data.Semigroup (Semigroup, (<>), )
 
@@ -32,7 +36,7 @@
 
 data CGMState = CGMState {
     cgm_module :: Module,
-    cgm_externs :: [(String, Function)],
+    cgm_externs :: Map String Function,
     cgm_global_mappings :: GlobalMappings,
     cgm_next :: !Int
     }
@@ -55,7 +59,7 @@
     evalStateT body $
     CGMState {
         cgm_module = m, cgm_next = 1,
-        cgm_externs = [], cgm_global_mappings = mempty
+        cgm_externs = Map.empty, cgm_global_mappings = mempty
     }
 
 --------------------------------------
@@ -86,14 +90,14 @@
 getFunctionModule :: CodeGenFunction a Module
 getFunctionModule = CGF $ gets (cgm_module . cgf_module)
 
-getExterns :: CodeGenFunction a [(String, Function)]
-getExterns = CGF $ gets (cgm_externs . cgf_module)
+lookupExtern :: String -> CodeGenFunction a (Maybe Function)
+lookupExtern name = CGF $ gets (Map.lookup name . cgm_externs . cgf_module)
 
-putExterns :: [(String, Function)] -> CodeGenFunction a ()
-putExterns es = do
-    cgf <- CGF get
-    let cgm' = (cgf_module cgf) { cgm_externs = es }
-    CGF $ put (cgf { cgf_module = cgm' })
+addExtern :: String -> Function -> CodeGenFunction a ()
+addExtern name func = CGF $ modify $ \cgf ->
+    cgf {cgf_module = (cgf_module cgf)
+            {cgm_externs =
+                Map.insert name func (cgm_externs $ cgf_module cgf) } }
 
 
 type Value = FFI.ValueRef
diff --git a/private/LLVM/Core/Instructions.hs b/private/LLVM/Core/Instructions.hs
--- a/private/LLVM/Core/Instructions.hs
+++ b/private/LLVM/Core/Instructions.hs
@@ -562,7 +562,7 @@
 -- |Acceptable arguments to 'extractvalue' and 'insertvalue'.
 class GetValue agg ix where
     type ValueType agg ix
-    getIx :: LP.Proxy agg -> ix -> CUInt
+    getIx :: proxy agg -> ix -> CUInt
 
 instance (GetField as i, Dec.Natural i) => GetValue (Struct as) (Proxy i) where
     type ValueType (Struct as) (Proxy i) = FieldType as i
@@ -592,11 +592,11 @@
              => Value agg                   -- ^ Aggregate
              -> i                           -- ^ Index into the aggregate
              -> CodeGenFunction r (Value (ValueType agg i))
-extractvalue (Value agg) i =
+extractvalue v@(Value agg) i =
     liftM Value $
     withCurrentBuilder $ \ bldPtr ->
       U.withEmptyCString $
-        FFI.buildExtractValue bldPtr agg (getIx (LP.Proxy :: LP.Proxy agg) i)
+        FFI.buildExtractValue bldPtr agg (getIx v i)
 
 -- | Insert a value into an aggregate, nondestructive.
 insertvalue :: forall r agg i.
@@ -605,11 +605,11 @@
             -> Value (ValueType agg i)     -- ^ Value to insert
             -> i                           -- ^ Index into the aggregate
             -> CodeGenFunction r (Value agg)
-insertvalue (Value agg) (Value e) i =
+insertvalue v@(Value agg) (Value e) i =
     liftM Value $
     withCurrentBuilder $ \ bldPtr ->
       U.withEmptyCString $
-        FFI.buildInsertValue bldPtr agg e (getIx (LP.Proxy :: LP.Proxy agg) i)
+        FFI.buildInsertValue bldPtr agg e (getIx v i)
 
 
 --------------------------------------
diff --git a/src/LLVM/Util/Arithmetic.hs b/src/LLVM/Util/Arithmetic.hs
--- a/src/LLVM/Util/Arithmetic.hs
+++ b/src/LLVM/Util/Arithmetic.hs
@@ -238,14 +238,14 @@
 
 instance (Value a ~ b) => ToArithFunction r (IO a) (CodeGenFunction r b) where
     type TFunA (CodeGenFunction r b) = IO (UnValue b)
-    type TFunB r (IO a) = CodeGenFunction r (Value a)
+    type TFunB r (IO a) = TValue r a
     toArithFunction' cl = runCall =<< cl
 
 instance
     (ToArithFunction r b0 b1, CodeGenFunction r (Value a0) ~ a1) =>
         ToArithFunction r (a0 -> b0) (a1 -> b1) where
     type TFunA (a1 -> b1) = UnValue (CodeValue a1) -> TFunA b1
-    type TFunB r (a0 -> b0) = CodeGenFunction r (Value a0) -> TFunB r b0
+    type TFunB r (a0 -> b0) = TValue r a0 -> TFunB r b0
     toArithFunction' cl x =
         toArithFunction' (liftM2 applyCall cl x)