From 8e12798a0560ee68a46d8a7b5b5d2050b3ff5f1e Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Mon, 11 Sep 2017 09:41:46 -0400
Subject: [PATCH] Define a let binding smart constructor using circular
 programming.

---
 src/Data/Functor/Binding.hs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/Data/Functor/Binding.hs b/src/Data/Functor/Binding.hs
index df043b5a7..8154917d0 100644
--- a/src/Data/Functor/Binding.hs
+++ b/src/Data/Functor/Binding.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE DerivingStrategies, GADTs, GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE DerivingStrategies, GADTs, GeneralizedNewtypeDeriving, NoStrictData #-}
 module Data.Functor.Binding
 ( Metavar(..)
 -- Abstract binding trees
@@ -6,6 +6,7 @@ module Data.Functor.Binding
 , bindings
 , freeMetavariables
 , maxBoundMetavariable
+, letBind
 -- Environments
 , Env(..)
 , envExtend
@@ -48,6 +49,12 @@ foldMaxMap :: (Foldable t, Ord b) => (a -> Maybe b) -> t a -> Maybe b
 foldMaxMap f = foldr (max . f) Nothing
 
 
+letBind :: (Foldable syntax, Functor syntax, Corecursive t, Recursive t, Base t ~ BindingF syntax) => t -> (Metavar -> syntax t) -> t
+letBind diff f = embed (Let [(n, diff)] body)
+  where body = f n
+        n = maybe (Metavar 0) succ (foldMaxMap maxBoundMetavariable body)
+
+
 newtype Env a = Env { unEnv :: [(Metavar, a)] }
   deriving (Eq, Foldable, Functor, Monoid, Ord, Show, Traversable)