diff --git a/.gitignore b/.gitignore
index fb1c996a9..d1e036b45 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,9 +9,6 @@ cabal-dev
 *.un~
 **/cache/**
 **/build/**
-store
-codestore
-codebase
 tags
 unison-src/.loaded
 **cabal.sandbox.config
@@ -20,6 +17,8 @@ blockstore
 blockstore.leveldb
 target
 htags
+scala-tags
+haskell-tags
 
 # Stack
 .stack-work
diff --git a/parser-typechecker/src/Unison/Codebase.hs b/parser-typechecker/src/Unison/Codebase.hs
new file mode 100644
index 000000000..b0ac5f5af
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase.hs
@@ -0,0 +1,24 @@
+module Unison.Codebase where
+
+import qualified Unison.Term as Term
+import qualified Unison.DataDeclaration as DD
+import Unison.Reference (Reference)
+import Unison.Codebase.Code (Code)
+import Unison.Codebase.Branch (Branch)
+import Unison.Codebase.Release (Release)
+
+type DataDeclaration v a = DD.DataDeclaration' v a
+type Term v a = Term.AnnotatedTerm v a
+type Name = String
+
+data Codebase m v a =
+  Codebase { getCode :: Reference -> m (Maybe (Code v a))
+           , putCode :: Code v a -> m Reference
+           , branches :: m [Name]
+           , getBranch :: Name -> m (Maybe (Branch v a))
+           , putBranch :: Name -> Branch v a -> m ()
+           , releases :: m [Name]
+           , getRelease :: Name -> m (Maybe (Release v a))
+           , putRelease :: Name -> Release v a -> m ()
+           }
+
diff --git a/parser-typechecker/src/Unison/Codebase/Branch.hs b/parser-typechecker/src/Unison/Codebase/Branch.hs
new file mode 100644
index 000000000..4f7296a71
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Branch.hs
@@ -0,0 +1,18 @@
+module Unison.Codebase.Branch where
+
+import Data.Map (Map)
+import Unison.Codebase.Causal (Causal)
+import Unison.Codebase.Conflicted (Conflicted)
+import Unison.Codebase.Name (Name)
+import Unison.Codebase.NameEdit (NameEdit)
+import Unison.Codebase.TermEdit (TermEdit)
+import Unison.Codebase.TypeEdit (TypeEdit)
+import Unison.Reference (Reference)
+
+data Branch v a =
+  Branch { namespace     :: Map Name (Causal NameEdit)
+         , edited        :: Map Reference (Causal (Conflicted TermEdit))
+         , editedDatas   :: Map Reference (Causal (Conflicted TypeEdit))
+         , editedEffects :: Map Reference (Causal (Conflicted TypeEdit))
+         }
+
diff --git a/parser-typechecker/src/Unison/Codebase/Causal.hs b/parser-typechecker/src/Unison/Codebase/Causal.hs
new file mode 100644
index 000000000..91e4fd5b7
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Causal.hs
@@ -0,0 +1,45 @@
+module Unison.Codebase.Causal where
+
+import Prelude hiding (head, sequence)
+import Data.List (sort)
+import Unison.Hash (Hash)
+import qualified Unison.Hashable as Hashable
+import Unison.Hashable (Hashable)
+
+data Causal e
+  = One { currentHash :: Hash, head :: e }
+  | Cons { currentHash :: Hash, head :: e, tail :: Causal e }
+  | Merge { currentHash :: Hash, head :: e, tail1 :: Causal e, tail2 :: Causal e }
+
+instance Semigroup e => Semigroup (Causal e) where
+  a <> b
+    | before a b = b
+    | before b a = a
+    | otherwise  = Merge (mixHashes [currentHash a, currentHash b])
+                         (head a <> head b) a b
+
+hash :: Hashable e => e -> Hash
+hash = Hashable.accumulate'
+
+-- commutative combine of a list of hashes
+mixHashes :: [Hash] -> Hash
+mixHashes = hash . sort
+
+one :: Hashable e => e -> Causal e
+one e = One (hash e) e
+
+cons :: Hashable e => e -> Causal e -> Causal e
+cons e tl = Cons (hash [hash e, currentHash tl]) e tl
+
+sequence :: (Semigroup e, Hashable e) => Causal e -> Causal e -> Causal e
+sequence a (One _ e) = cons e a
+sequence a (Cons _ e tl) = cons e (sequence a tl)
+sequence a (Merge _ _ l r) = sequence a l <> r
+-- note: if causal had a `split` operation, we'd need to sequence on both sides
+
+-- Does `h2` incorporate all of `h1`?
+before :: Causal e -> Causal e -> Bool
+before h1 h2 = go (currentHash h1) h2 where
+  go h1 (One h _) = h == h1
+  go h1 (Cons h _ tl) = h == h1 || go h1 tl
+  go h1 (Merge h _ left right) = h == h1 || go h1 left || go h1 right
diff --git a/parser-typechecker/src/Unison/Codebase/Code.hs b/parser-typechecker/src/Unison/Codebase/Code.hs
new file mode 100644
index 000000000..4325d1bcd
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Code.hs
@@ -0,0 +1,12 @@
+module Unison.Codebase.Code where
+
+import qualified Unison.Term as Term
+import qualified Unison.DataDeclaration as DD
+
+type DataDeclaration v a = DD.DataDeclaration' v a
+type Term v a = Term.AnnotatedTerm v a
+
+data Code v a
+  = Term (Term v a)
+  | DataDeclaration (DataDeclaration v a)
+
diff --git a/parser-typechecker/src/Unison/Codebase/Conflicted.hs b/parser-typechecker/src/Unison/Codebase/Conflicted.hs
new file mode 100644
index 000000000..52bd00658
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Conflicted.hs
@@ -0,0 +1,12 @@
+module Unison.Codebase.Conflicted where
+
+import qualified Data.Set as Set
+import Data.Set (Set)
+
+data Conflicted a = One a | Many (Set a)
+
+instance Ord a => Semigroup (Conflicted a) where
+  One a <> One a2 = if a == a2 then One a else Many (Set.fromList [a,a2])
+  One a <> Many as = Many (Set.insert a as)
+  Many as <> One a = Many (Set.insert a as)
+  Many as <> Many as2 = Many (as `Set.union` as2)
diff --git a/parser-typechecker/src/Unison/Codebase/Name.hs b/parser-typechecker/src/Unison/Codebase/Name.hs
new file mode 100644
index 000000000..1c041eac5
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Name.hs
@@ -0,0 +1,3 @@
+module Unison.Codebase.Name where
+
+type Name = String
diff --git a/parser-typechecker/src/Unison/Codebase/NameEdit.hs b/parser-typechecker/src/Unison/Codebase/NameEdit.hs
new file mode 100644
index 000000000..9e257a32a
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/NameEdit.hs
@@ -0,0 +1,7 @@
+module Unison.Codebase.NameEdit where
+
+import Data.Set (Set)
+import Unison.Reference (Reference)
+
+data NameEdit =
+  NameEdit { added :: Set Reference, removed :: Set Reference }
diff --git a/parser-typechecker/src/Unison/Codebase/Release.hs b/parser-typechecker/src/Unison/Codebase/Release.hs
new file mode 100644
index 000000000..148008993
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/Release.hs
@@ -0,0 +1,21 @@
+module Unison.Codebase.Release where
+
+import Data.Map (Map)
+import Unison.Codebase.Code (Code)
+import Unison.Codebase.Name (Name)
+import Unison.Codebase.TermEdit (TermEdit)
+import Unison.Codebase.TypeEdit (TypeEdit)
+import Unison.Reference (Reference)
+import qualified Unison.DataDeclaration as DD
+import qualified Unison.Term as Term
+
+type DataDeclaration v a = DD.DataDeclaration' v a
+type Term v a = Term.AnnotatedTerm v a
+
+data Release v a =
+  Release { namespace     :: Map Name (Code v a)
+          , edited        :: Map Reference TermEdit
+          , editedDatas   :: Map Reference TypeEdit
+          , editedEffects :: Map Reference TypeEdit }
+
+
diff --git a/parser-typechecker/src/Unison/Codebase/TermEdit.hs b/parser-typechecker/src/Unison/Codebase/TermEdit.hs
new file mode 100644
index 000000000..060e50e9c
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/TermEdit.hs
@@ -0,0 +1,10 @@
+module Unison.Codebase.TermEdit where
+
+import Unison.Reference (Reference)
+
+data TermEdit = Replace Reference Typing | Deprecate
+
+-- Replacements with the Same type can be automatically propagated.
+-- Replacements with a Subtype can be automatically propagated but may result in dependents getting more general types, so requires re-inference.
+-- Replacements of a Different type need to be manually propagated by the programmer.
+data Typing = Same | Subtype | Different
diff --git a/parser-typechecker/src/Unison/Codebase/TypeEdit.hs b/parser-typechecker/src/Unison/Codebase/TypeEdit.hs
new file mode 100644
index 000000000..1eaed639e
--- /dev/null
+++ b/parser-typechecker/src/Unison/Codebase/TypeEdit.hs
@@ -0,0 +1,5 @@
+module Unison.Codebase.TypeEdit where
+
+import Unison.Reference (Reference)
+
+data TypeEdit = Replace Reference | Deprecated
diff --git a/parser-typechecker/src/Unison/Hash.hs b/parser-typechecker/src/Unison/Hash.hs
index d1d816da8..eb5a374c7 100644
--- a/parser-typechecker/src/Unison/Hash.hs
+++ b/parser-typechecker/src/Unison/Hash.hs
@@ -23,6 +23,9 @@ newtype Hash = Hash { toBytes :: ByteString } deriving (Eq,Ord,Generic)
 instance Show Hash where
   show h = take 8 $ Text.unpack (base58 h)
 
+instance H.Hashable Hash where
+  tokens h = [H.Bytes (toBytes h)]
+
 fromBytesImpl :: ByteString -> Hash
 fromBytesImpl = fromBytes
 
diff --git a/parser-typechecker/unison-parser-typechecker.cabal b/parser-typechecker/unison-parser-typechecker.cabal
index 0e65b43f2..7398139d0 100644
--- a/parser-typechecker/unison-parser-typechecker.cabal
+++ b/parser-typechecker/unison-parser-typechecker.cabal
@@ -38,6 +38,16 @@ library
     Unison.Blank
     Unison.Builtin
     Unison.Codecs
+    Unison.Codebase
+    Unison.Codebase.Branch
+    Unison.Codebase.Causal
+    Unison.Codebase.Code
+    Unison.Codebase.Conflicted
+    Unison.Codebase.Name
+    Unison.Codebase.NameEdit
+    Unison.Codebase.Release
+    Unison.Codebase.TermEdit
+    Unison.Codebase.TypeEdit
     Unison.DataDeclaration
     Unison.FileParser
     Unison.FileParsers