From dfad3a25c306b06873a436d4b8ce7d764bf29005 Mon Sep 17 00:00:00 2001
From: Eric Bailey <eric@ericb.me>
Date: Tue, 16 Feb 2016 03:55:52 -0600
Subject: [PATCH 1/4] Update #'cats.core.monad.maybe/just

Remove nullary clause, since that doesn't make much sense anyway, and
add a precondition to the remaining unary clause to ensure v is non-nil.

Update the docsctring accordingly.
---
 src/cats/monad/maybe.cljc | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/src/cats/monad/maybe.cljc b/src/cats/monad/maybe.cljc
index 7c7efd3..ce53a7b 100644
--- a/src/cats/monad/maybe.cljc
+++ b/src/cats/monad/maybe.cljc
@@ -111,12 +111,10 @@
     false))
 
 (defn just
-  "A Just type constructor.
-
-  Without arguments it returns a Just instance
-  with nil as wrapped value."
-  ([] (Just. nil))
-  ([v] (Just. v)))
+  "A Just type constructor."
+  [v]
+  {:pre [(some? v)]}
+  (Just. v))
 
 (defn nothing
   "A Nothing type constructor."

From 3c57373e052112d9c8a49e8c2b768c4544161152 Mon Sep 17 00:00:00 2001
From: Eric Bailey <eric@ericb.me>
Date: Tue, 16 Feb 2016 04:22:20 -0600
Subject: [PATCH 2/4] Update cats.monad.maybe

In -mreturn, if v is nil, return #<Nothing>, otherwise #<Just v>.
---
 src/cats/monad/maybe.cljc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/cats/monad/maybe.cljc b/src/cats/monad/maybe.cljc
index ce53a7b..c9650ce 100644
--- a/src/cats/monad/maybe.cljc
+++ b/src/cats/monad/maybe.cljc
@@ -202,7 +202,9 @@
 
     p/Monad
     (-mreturn [_ v]
-      (just v))
+      (if (nil? v)
+        (nothing)
+        (just v)))
     (-mbind [_ mv f]
       (if (nothing? mv)
         mv

From 1eed32347668e1a4cedb9e476a945dcda49f1960 Mon Sep 17 00:00:00 2001
From: Eric Bailey <eric@ericb.me>
Date: Tue, 16 Feb 2016 04:23:11 -0600
Subject: [PATCH 3/4] Update #'cats.core/guard

(return b) instead of (return nil)

This is mostly so the maybe tests pass and could be very misguided.
---
 src/cats/core.cljc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/cats/core.cljc b/src/cats/core.cljc
index 7d4f7de..0b20ea3 100644
--- a/src/cats/core.cljc
+++ b/src/cats/core.cljc
@@ -110,7 +110,7 @@
 (defn guard
   [b]
   (if b
-    (return nil)
+    (return b)
     (mzero)))
 
 (defn join

From 0a2b4981a9c9444e0e3ad4a598ba9c06fb9cd31e Mon Sep 17 00:00:00 2001
From: Eric Bailey <eric@ericb.me>
Date: Tue, 16 Feb 2016 04:53:00 -0600
Subject: [PATCH 4/4] Update #'cats.core/guard

(return true) per @dialelo's suggestion.

https://github.com/funcool/cats/pull/149#issuecomment-184622726
---
 src/cats/core.cljc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/cats/core.cljc b/src/cats/core.cljc
index 0b20ea3..c05f8d1 100644
--- a/src/cats/core.cljc
+++ b/src/cats/core.cljc
@@ -110,7 +110,7 @@
 (defn guard
   [b]
   (if b
-    (return b)
+    (return true)
     (mzero)))
 
 (defn join