From bdc819cda86683183533e39ef3b4aff5399efc67 Mon Sep 17 00:00:00 2001
From: Karl Ostmo <kostmo@gmail.com>
Date: Wed, 19 Jul 2023 14:41:01 -0700
Subject: [PATCH] address sequenceTuple issue

---
 .../Scenario/Topography/Navigation/Portal.hs  | 20 ++++++++++++++++---
 src/Swarm/Util.hs                             |  2 +-
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs b/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
index 143883d92f..d74e085367 100644
--- a/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
+++ b/src/Swarm/Game/Scenario/Topography/Navigation/Portal.hs
@@ -257,22 +257,36 @@ ensureSpatialConsistency xs =
   nonUniform :: Map (SubworldName, SubworldName) (NonEmpty (V2 Int32))
   nonUniform = M.filter ((not . allEqual) . NE.toList) vectorized
 
--- | This signature looks a lot like the Traversable instance:
+-- |
+-- An implementation of 'sequenceA' for 'Signed' that does not
+-- require an 'Applicative' instance for the inner 'Functor'.
+--
+-- == Discussion
+-- Compare to the 'Traversable' instance of 'Signed':
 -- @
 -- instance Traversable Signed where
 --   traverse f (Positive x) = Positive <$> f x
 --   traverse f (Negative x) = Negative <$> f x
 -- @
 --
--- if we were to substitute "id" for f:
+-- if we were to substitute 'id' for f:
 -- @
 --   traverse id (Positive x) = Positive <$> id x
 --   traverse id (Negative x) = Negative <$> id x
 -- @
+-- our implementation essentially becomes @traverse id@.
+--
+-- However, we cannot simply write our implementation as @traverse id@, because
+-- the 'traverse' function has an 'Applicative' constraint, which is superfluous
+-- for our purpose.
+--
+-- Perhaps there is an opportunity to invent a typeclass for datatypes which
+-- consist exclusively of unary (or more ambitiously, non-nullary?) data constructors,
+-- for which a less-constrained 'sequence' function could be automatically derived.
 sequenceSigned ::
   Functor f =>
   Signed (f a) ->
   f (Signed a)
 sequenceSigned = \case
   Positive x -> Positive <$> x
-  Negative x -> Negative <$> x
\ No newline at end of file
+  Negative x -> Negative <$> x
diff --git a/src/Swarm/Util.hs b/src/Swarm/Util.hs
index 07c1af4b0c..e71a8af8d5 100644
--- a/src/Swarm/Util.hs
+++ b/src/Swarm/Util.hs
@@ -22,7 +22,7 @@ module Swarm.Util (
   findDup,
   both,
   allEqual,
-  
+
   -- * Directory utilities
   readFileMay,
   readFileMayT,