pretty printer

uses ISeq, as described in 'Implementing Functional Languages'. going to try using a style similar to ShowS instead.

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