diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 0.5.0.0
+
+* Fix dodgy `Bluefin.Primitive.primitive` implementation
+
 # 0.4.3.0
 
 * Add `Bluefin.GadtEffect`
diff --git a/bluefin.cabal b/bluefin.cabal
--- a/bluefin.cabal
+++ b/bluefin.cabal
@@ -1,6 +1,6 @@
 cabal-version:      3.0
 name:               bluefin
-version:            0.4.3.0
+version:            0.5.0.0
 license:            MIT
 license-file:       LICENSE
 author:             Tom Ellis
@@ -45,6 +45,6 @@
       Bluefin.System.IO,
       Bluefin.Writer,
     build-depends:
-      bluefin-internal >= 0.4.3.0 && < 0.5
+      bluefin-internal >= 0.4.3.0 && < 0.6
     hs-source-dirs:   src
     default-language: Haskell2010
diff --git a/src/Bluefin/Prim.hs b/src/Bluefin/Prim.hs
--- a/src/Bluefin/Prim.hs
+++ b/src/Bluefin/Prim.hs
@@ -2,7 +2,7 @@
 --
 -- @
 -- -- Define a handle which includes Prim
--- data ExAndPrim e = MkExAndPrim (Exception String e) (Prim e)
+-- data ExAndPrim e = MkExAndPrim (Exception String e) (P.Prim e)
 --   -- Give it a Handle instance, as per Bluefin.Compound
 --   deriving (Handle) via OneWayCoercibleHandle ExAndPrim
 --   deriving stock (Generic)
@@ -11,15 +11,34 @@
 --   oneWayCoercibleImpl = gOneWayCoercible
 --
 -- -- Define a monad M containing the Prim handle
--- newtype M es a = MkM (DslBuilderEff ExAndPrim es a)
+-- newtype M e es a = MkM (ReaderT (ExAndPrim e) (Eff es) a)
 --   deriving newtype (Functor, Applicative, Monad)
 --
+-- -- Define a way of running M
+-- runM ::
+--   (e1 :> es, e2 :> es) =>
+--   Exception String e1 ->
+--   P.Prim e2 ->
+--   M es es r ->
+--   Eff es r
+-- runM ex prim (MkM m) =
+--   runReaderT m (MkExAndPrim (mapHandle ex) (mapHandle prim))
+--
 -- -- Give M a PrimMonad instance
--- instance PrimMonad (M es) where
---   type PrimState (M es) = PrimStateEff es
+-- instance (e :> es) => PrimMonad (M e es) where
+--   type PrimState (M e es) = P.PrimStateEff e
 --   primitive f =
---     MkM (dslBuilderEff (\\(MkExAndPrim _ prim) -> P.primitive prim f))
+--     MkM (ReaderT (\\(MkExAndPrim _ prim) -> P.'primitive' prim f))
+--
+-- -- ghci> example
+-- -- Right [\"Hello\",\"World\"]
+-- example :: Either String [String]
+-- example = runPureEff $ try $ \\ex -> P.'runPrim' $ \\prim -> do
+--   runM ex prim $ do
+--     arr <- A.newArray 2 \"Hello\"
+--     A.writeArray arr 1 \"World\"
+--     for [0, 1] (A.readArray arr)
 -- @
-module Bluefin.Prim (Prim, PrimStateEff, primitive) where
+module Bluefin.Prim (Prim, runPrim, PrimStateEff, primitive) where
 
 import Bluefin.Internal.Prim