diff --git a/rlp.cabal b/rlp.cabal
index 9d9120a..ad5cca8 100644
--- a/rlp.cabal
+++ b/rlp.cabal
@@ -22,6 +22,7 @@ library
                     , TI
     other-modules:    Data.Heap
                     , Control.DFA
+                    , Data.Pretty
 
     -- other-extensions:
     build-depends:    base ^>=4.18.0.0
diff --git a/src/Core.hs b/src/Core.hs
index 6ab02c7..f821841 100644
--- a/src/Core.hs
+++ b/src/Core.hs
@@ -2,6 +2,7 @@
 module Core where
 ----------------------------------------------------------------------------------
 import Data.Coerce
+import Data.Pretty
 ----------------------------------------------------------------------------------
 
 data Expr = Var Name
@@ -39,6 +40,11 @@ newtype Program = Program [ScDef]
 
 ----------------------------------------------------------------------------------
 
+instance Pretty Expr where
+    prettyPrec _ (Var k) = iStr k
+
+----------------------------------------------------------------------------------
+
 instance Semigroup Program where
     (<>) = coerce $ (++) @ScDef
 
diff --git a/src/Data/Pretty.hs b/src/Data/Pretty.hs
new file mode 100644
index 0000000..52c9b2b
--- /dev/null
+++ b/src/Data/Pretty.hs
@@ -0,0 +1,54 @@
+module Data.Pretty
+    ( Pretty(..)
+    , ISeq
+    , iNil
+    , iStr
+    , iAppend
+    )
+    where
+----------------------------------------------------------------------------------
+
+class Pretty a where
+    pretty :: a -> String
+    prettyPrec :: Int -> a -> ISeq
+
+    pretty = squash . prettyPrec 0
+    prettyPrec _ a = iBracket (iStr $ pretty a)
+
+data ISeq where
+    INil :: ISeq
+    IStr :: String -> ISeq
+    IAppend :: ISeq -> ISeq -> ISeq
+
+instance Semigroup ISeq where
+    (<>) = IAppend
+
+instance Monoid ISeq where
+    mempty = INil
+
+squash :: ISeq -> String
+squash = flatten . pure
+
+flatten :: [ISeq] -> String
+flatten (INil : ss)        = flatten ss
+flatten (IStr s : ss)      = s ++ flatten ss
+flatten (IAppend r s : ss) = flatten (r : s : ss)
+
+iNil :: ISeq
+iNil = INil
+
+iStr :: String -> ISeq
+iStr = IStr
+
+iAppend  :: ISeq -> ISeq -> ISeq
+iAppend = IAppend
+
+iIndent :: ISeq -> ISeq
+iIndent = id
+
+iBreak :: ISeq
+iBreak = iStr "\n"
+
+iBracket :: ISeq -> ISeq
+iBracket s = iStr "(" `iAppend` s `iAppend` iStr ")"
+