Simplify runTermE (runtime-wise) (#63)

* simplify runTermE

* explain what we're doing

Author: MaximilianAlgehed

src/Constrained/AbstractSyntax.hs

@@ -104,6 +104,10 @@ runTermE env = \case
   V v -> case Env.lookup env v of
     Just a -> Right a
     Nothing -> Left (pure ("Couldn't find " ++ show v ++ " in " ++ show env))
+  -- The first two cases here are an optimization to avoid dispatching to `mapMList` (which does all sorts of
+  -- unpacking and packing and doesn't fuse nicely with `uncurryList_`)
+  App f (ta :> Nil) -> semantics f <$> runTermE env ta
+  App f (ta :> tb :> Nil) -> semantics f <$> runTermE env ta <*> runTermE env tb
   App f ts -> do
     vs <- mapMList (fmap Identity . runTermE env) ts
     pure $ uncurryList_ runIdentity (semantics f) vs