From 19febc23aab6687120ea61b76027ce4c0455d06c Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Sat, 9 Sep 2017 18:03:07 +0100
Subject: [PATCH] Define diffCost as a catamorphism.

---
 src/Interpreter.hs | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/Interpreter.hs b/src/Interpreter.hs
index c35c55af1..f8b0308de 100644
--- a/src/Interpreter.hs
+++ b/src/Interpreter.hs
@@ -11,6 +11,7 @@ import Control.Monad.Free.Freer
 import Data.Align.Generic
 import Data.Functor.Both
 import Data.Functor.Classes (Eq1)
+import Data.Functor.Foldable (cata)
 import Data.Hashable (Hashable)
 import Data.Maybe (isJust)
 import Data.Record
@@ -124,9 +125,8 @@ defaultM = 10
 -- | Computes a constant-time approximation to the edit distance of a diff. This is done by comparing at most _m_ nodes, & assuming the rest are zero-cost.
 editDistanceUpTo :: (GAlign f, Foldable f, Functor f) => Integer -> These (Term f (Record fields)) (Term f (Record fields)) -> Int
 editDistanceUpTo m = these termSize termSize (\ a b -> diffCost m (approximateDiff a b))
-  where diffCost m (Diff.Diff diff)
-          | m <= 0    = 0
-          | otherwise = case diff of
-            Copy _ r -> sum (fmap (diffCost (pred m)) r)
-            Patch patch -> patchSum termSize patch
+  where diffCost = flip . cata $ \ diff m -> case diff of
+          _ | m <= 0 -> 0
+          Copy _ r -> sum (fmap ($ pred m) r)
+          Patch patch -> patchSum termSize patch
         approximateDiff a b = maybe (replacing a b) (copy (both (extract a) (extract b))) (galignWith (these deleting inserting approximateDiff) (unwrap a) (unwrap b))