From ac7f187d57f0e6af9c0fb2540990e0e1b5d341e9 Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Mon, 16 Dec 2019 19:17:01 +0100
Subject: [PATCH 1/6] Added documentation to all the affected functions of
 Issue https://github.com/elm/core/issues/1072#issuecomment-565860545 to warn
 affected developers.

---
 src/Basics.elm | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/src/Basics.elm b/src/Basics.elm
index fa05e402..cdb8b5e7 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -199,6 +199,14 @@ mul =
     -1 / 4 == -0.25
     -5 / 4 == -1.25
 
+**Be careful:**
+
+    4 / 0 == Infinity : Float
+    3 / 0 == Infinity : Float
+
+This is a bug and Infinity is not a legitimate value
+in Elm, thus you can't match for it.
+You'll have to check beforehand wheter the divisor is 0
 -}
 fdiv : Float -> Float -> Float
 fdiv =
@@ -221,6 +229,13 @@ similar to `truncate (3 / 4)`.
 
 It may sometimes be useful to pair this with the [`remainderBy`](#remainderBy)
 function.
+
+Be careful:
+
+    4 // 0 == 0
+
+This is a bug and you'll have to check before computing the result,
+if your divisor is 0.
 -}
 idiv : Int -> Int -> Int
 idiv =
@@ -535,6 +550,15 @@ or read Daan Leijen’s [Division and Modulus for Computer Scientists][dm] for m
 information.
 
 [dm]: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/divmodnote-letter.pdf
+
+Be careful:
+
+    modBy 0 5
+
+results in a crash, a **runtime exception** of the sort:
+"Error: Cannot perform mod 0. Division by zero error."
+You'll have to check, that the first argument isn't 0.
+Needless to say, this is a bug!
 -}
 modBy : Int -> Int -> Int
 modBy =
@@ -551,6 +575,13 @@ or read Daan Leijen’s [Division and Modulus for Computer Scientists][dm] for m
 information.
 
 [dm]: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/divmodnote-letter.pdf
+
+Be careful:
+
+    remainderBy 0 5 == NaN : Int
+
+This is a bug in Elm and `NaN` isn't a legitimate value.
+You'll have to check beforehand, that the first argument isn't 0.
 -}
 remainderBy : Int -> Int -> Int
 remainderBy =

From b53eaf335298e8aebe1c94e9d23fcf61e1f03660 Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Mon, 16 Dec 2019 19:20:07 +0100
Subject: [PATCH 2/6] Consistent attention seeking.

---
 src/Basics.elm | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/Basics.elm b/src/Basics.elm
index cdb8b5e7..5f093322 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -230,7 +230,7 @@ similar to `truncate (3 / 4)`.
 It may sometimes be useful to pair this with the [`remainderBy`](#remainderBy)
 function.
 
-Be careful:
+**Be careful:**
 
     4 // 0 == 0
 
@@ -551,7 +551,7 @@ information.
 
 [dm]: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/divmodnote-letter.pdf
 
-Be careful:
+**Be careful:**
 
     modBy 0 5
 
@@ -576,7 +576,7 @@ information.
 
 [dm]: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/divmodnote-letter.pdf
 
-Be careful:
+**Be careful:**
 
     remainderBy 0 5 == NaN : Int
 

From 55fc7baf09f2868b160785431b6bb9a8e0f80da8 Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Fri, 17 Jan 2020 16:14:18 +0100
Subject: [PATCH 3/6] Changed fdiv function, to use Maybe - returning Nothing,
 if divisor = 0.

---
 src/Basics.elm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/Basics.elm b/src/Basics.elm
index 5f093322..253de3d9 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -208,9 +208,10 @@ This is a bug and Infinity is not a legitimate value
 in Elm, thus you can't match for it.
 You'll have to check beforehand wheter the divisor is 0
 -}
-fdiv : Float -> Float -> Float
-fdiv =
-  Elm.Kernel.Basics.fdiv
+fdiv : Float -> Float -> Maybe Float
+fdiv x divisor = case divisor of
+    0 -> Nothing
+    y -> Elm.Kernel.Basics.fdiv x y
 
 
 {-| Integer division:

From 07a9a29d2f842a766d45a2a9580cf6ac138a4e7f Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Fri, 17 Jan 2020 16:15:26 +0100
Subject: [PATCH 4/6] idiv function uses Maybe and returns Nothing if divisor =
 0.

---
 src/Basics.elm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/Basics.elm b/src/Basics.elm
index 253de3d9..9c5ccdf1 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -238,9 +238,10 @@ function.
 This is a bug and you'll have to check before computing the result,
 if your divisor is 0.
 -}
-idiv : Int -> Int -> Int
-idiv =
-  Elm.Kernel.Basics.idiv
+idiv : Int -> Int -> Maybe Int
+idiv x divisor = case divisor of
+    0 -> Nothing
+    y -> Elm.Kernel.Basics.idiv x y
 
 
 {-| Exponentiation

From e6c66097cbe450a30a4195e6e7f87b9ea7ced7d6 Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Fri, 17 Jan 2020 16:17:10 +0100
Subject: [PATCH 5/6] modby function uses Maybe and returns Nothing if divisor
 = 0. **This prevents a crash**

---
 src/Basics.elm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/Basics.elm b/src/Basics.elm
index 9c5ccdf1..28e2b72b 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -562,9 +562,10 @@ results in a crash, a **runtime exception** of the sort:
 You'll have to check, that the first argument isn't 0.
 Needless to say, this is a bug!
 -}
-modBy : Int -> Int -> Int
-modBy =
-  Elm.Kernel.Basics.modBy
+modBy : Int -> Int -> Maybe Int
+modBy x divisor = case divisor of
+  0 -> Nothing
+  y -> Elm.Kernel.Basics.modBy x y
 
 
 {-| Get the remainder after division. Here are bunch of examples of dividing by four:

From 3b1967a77ae4e54835a5a3765721174c138b755c Mon Sep 17 00:00:00 2001
From: Joris Gutjahr <joris.gutjahr@protonmail.com>
Date: Fri, 17 Jan 2020 16:19:26 +0100
Subject: [PATCH 6/6] remainderBy uses Maybe and returns Nothing if divisor =
 0. SOLVED small error.

---
 src/Basics.elm | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/Basics.elm b/src/Basics.elm
index 28e2b72b..fb0684dd 100644
--- a/src/Basics.elm
+++ b/src/Basics.elm
@@ -211,7 +211,7 @@ You'll have to check beforehand wheter the divisor is 0
 fdiv : Float -> Float -> Maybe Float
 fdiv x divisor = case divisor of
     0 -> Nothing
-    y -> Elm.Kernel.Basics.fdiv x y
+    y -> Just <| Elm.Kernel.Basics.fdiv x y
 
 
 {-| Integer division:
@@ -241,7 +241,7 @@ if your divisor is 0.
 idiv : Int -> Int -> Maybe Int
 idiv x divisor = case divisor of
     0 -> Nothing
-    y -> Elm.Kernel.Basics.idiv x y
+    y -> Just <| Elm.Kernel.Basics.idiv x y
 
 
 {-| Exponentiation
@@ -565,7 +565,7 @@ Needless to say, this is a bug!
 modBy : Int -> Int -> Maybe Int
 modBy x divisor = case divisor of
   0 -> Nothing
-  y -> Elm.Kernel.Basics.modBy x y
+  y -> Just <| Elm.Kernel.Basics.modBy x y
 
 
 {-| Get the remainder after division. Here are bunch of examples of dividing by four:
@@ -586,9 +586,10 @@ information.
 This is a bug in Elm and `NaN` isn't a legitimate value.
 You'll have to check beforehand, that the first argument isn't 0.
 -}
-remainderBy : Int -> Int -> Int
-remainderBy =
-  Elm.Kernel.Basics.remainderBy
+remainderBy : Int -> Int -> Maybe Int
+remainderBy x divisor = case divisor of
+    0 -> Nothing
+    y -> Just <| Elm.Kernel.Basics.remainderBy x y
 
 
 {-| Negate a number.