From 797330650f71c62c4d93685b33a8af7cebc52d01 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 29 Nov 2017 13:07:15 -0500
Subject: [PATCH] Customize the default eval to show the constructor.

---
 src/Abstract/Eval.hs | 5 +++--
 src/Data/Syntax.hs   | 1 -
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/Abstract/Eval.hs b/src/Abstract/Eval.hs
index 5f1afa1dd..acad379d1 100644
--- a/src/Abstract/Eval.hs
+++ b/src/Abstract/Eval.hs
@@ -12,6 +12,7 @@ import Abstract.Value
 import Control.Monad.Effect
 import Control.Monad.Effect.Reader
 import Control.Monad.Fail
+import Data.Functor.Classes
 import Data.Proxy
 import Data.Semigroup
 import qualified Data.Set as Set
@@ -23,8 +24,8 @@ import Prelude hiding (fail)
 -- Collecting evaluator
 class Monad m => Eval v m constr where
   eval :: FreeVariables term => ((v -> m v) -> term -> m v) ->  (v -> m w) -> constr term -> m w
-  default eval :: (FreeVariables term, MonadFail m) => ((v -> m v) -> term -> m v) ->  (v -> m w) -> constr term -> m w
-  eval _ _ _ = fail "default eval"
+  default eval :: (FreeVariables term, MonadFail m, Show1 constr) => ((v -> m v) -> term -> m v) ->  (v -> m w) -> constr term -> m w
+  eval _ _ expr = fail $ "Eval unspecialized for " ++ liftShowsPrec (const (const id)) (const id) 0 expr ""
 
 instance (Monad m, Apply (Eval v m) fs) => Eval v m (Union fs) where
   eval ev yield = apply (Proxy :: Proxy (Eval v m)) (eval ev yield)
diff --git a/src/Data/Syntax.hs b/src/Data/Syntax.hs
index b476af5a6..c1bb1c0ec 100644
--- a/src/Data/Syntax.hs
+++ b/src/Data/Syntax.hs
@@ -183,7 +183,6 @@ instance Eq1 Error where liftEq = genericLiftEq
 instance Ord1 Error where liftCompare = genericLiftCompare
 instance Show1 Error where liftShowsPrec = genericLiftShowsPrec
 
--- TODO: Define Value semantics for Error
 instance (MonadFail m) => Eval v m Error
 
 errorSyntax :: Error.Error String -> [a] -> Error a