From 9c6a643c849e61f02265fbef1757bdb4b3ac8be8 Mon Sep 17 00:00:00 2001
From: Jorge Acereda <jacereda@gmail.com>
Date: Thu, 10 Nov 2016 23:28:38 +0100
Subject: [PATCH 1/2] Fix toEnum for (Maybe a)

---
 src/Data/Enum.purs       | 4 ++--
 test/Test/Data/Enum.purs | 9 ++++++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/Data/Enum.purs b/src/Data/Enum.purs
index 71d41f3..e630405 100644
--- a/src/Data/Enum.purs
+++ b/src/Data/Enum.purs
@@ -196,8 +196,8 @@ instance boundedEnumMaybe :: BoundedEnum a => BoundedEnum (Maybe a) where
   toEnum = to cardinality
     where
     to :: Cardinality a -> Int -> Maybe (Maybe a)
-    to _ 0 = Nothing
-    to (Cardinality ca) n | n <= ca = Just $ toEnum (n - 1)
+    to _ 0 = Just Nothing
+    to (Cardinality ca) n | n > 0 && n <= ca = Just $ toEnum (n - 1)
     to _ _ = Nothing
   fromEnum Nothing = 0
   fromEnum (Just e) = fromEnum e + 1
diff --git a/test/Test/Data/Enum.purs b/test/Test/Data/Enum.purs
index c57b50f..52b413f 100644
--- a/test/Test/Data/Enum.purs
+++ b/test/Test/Data/Enum.purs
@@ -7,7 +7,7 @@ import Control.Monad.Eff.Console (CONSOLE, log)
 
 import Data.Enum (class Enum, class BoundedEnum, defaultToEnum, defaultFromEnum,
                   defaultCardinality, enumFromTo, enumFromThenTo, upFrom,
-                  downFrom)
+                  downFrom, toEnum)
 import Data.Maybe (Maybe(..))
 
 import Test.Assert (ASSERT, assert)
@@ -63,3 +63,10 @@ testEnum = do
   assert $ downFrom D == [C, B, A]
   assert $ downFrom B == [      A]
   assert $ downFrom A == [       ]
+
+  log "BoundedEnum (Maybe Boolean)"
+  assert $ toEnum (-1) == Nothing :: Maybe (Maybe Boolean)
+  assert $ toEnum 0 == Just Nothing :: Maybe (Maybe Boolean)
+  assert $ toEnum 1 == Just (Just false) :: Maybe (Maybe Boolean)
+  assert $ toEnum 2 == Just (Just true) :: Maybe (Maybe Boolean)
+  assert $ toEnum 3 == Nothing :: Maybe (Maybe Boolean)

From 6c8fb737cc371796aaa8463e04379637c6ceb272 Mon Sep 17 00:00:00 2001
From: Jorge Acereda <jacereda@gmail.com>
Date: Thu, 10 Nov 2016 23:38:11 +0100
Subject: [PATCH 2/2] Simplify

---
 src/Data/Enum.purs | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/src/Data/Enum.purs b/src/Data/Enum.purs
index e630405..2939698 100644
--- a/src/Data/Enum.purs
+++ b/src/Data/Enum.purs
@@ -193,12 +193,8 @@ instance boundedEnumOrdering :: BoundedEnum Ordering where
 
 instance boundedEnumMaybe :: BoundedEnum a => BoundedEnum (Maybe a) where
   cardinality = Cardinality $ unwrap (cardinality :: Cardinality a) + 1
-  toEnum = to cardinality
-    where
-    to :: Cardinality a -> Int -> Maybe (Maybe a)
-    to _ 0 = Just Nothing
-    to (Cardinality ca) n | n > 0 && n <= ca = Just $ toEnum (n - 1)
-    to _ _ = Nothing
+  toEnum 0 = pure Nothing
+  toEnum n = Just <$> toEnum (n - 1)
   fromEnum Nothing = 0
   fromEnum (Just e) = fromEnum e + 1