Add message for GHC-06201

Author: fendor

message-index/messages/GHC-06201/example1/after/A.hs

@@ -0,0 +1,9 @@
+module A where
+
+class Calculator a where
+    add :: a -> a -> a
+    multiply :: a -> a -> a
+
+instance Calculator Int where
+    add a b = a + b
+    multiply a b = a * b

message-index/messages/GHC-06201/example1/before/A.hs

@@ -0,0 +1,8 @@
+module A where
+
+class Calculator a where
+    add :: a -> a -> a
+    multiply :: a -> a -> a
+
+instance Calculator Int where
+    add a b = a + b

message-index/messages/GHC-06201/example1/index.md

@@ -0,0 +1,20 @@
+---
+title: No explicit implementation for method instance
+---
+
+## Error Message
+
+```
+A.hs:7:10: warning: [GHC-06201] [-Wmissing-methods]
+    • No explicit implementation for
+        ‘multiply’
+    • In the instance declaration for ‘Calculator Int’
+  |
+7 | instance Calculator Int where
+```
+
+## Explanation
+
+The type class `Calculator` requires you to implement two methods: `add` and `multiply`.
+However, the example instance `instance Calculator Int` only implements the method `add` and not `multiply`
+To fix this, implement the method `multiply`!

message-index/messages/GHC-06201/example2/after/A.hs

@@ -0,0 +1,10 @@
+module A where
+
+data Box t = SomeBox t
+
+instance Functor Box where
+    fmap f (SomeBox a) = SomeBox (f a)
+
+instance Applicative Box where
+    pure a = SomeBox a
+    SomeBox f <*> SomeBox a = SomeBox (f a)

message-index/messages/GHC-06201/example2/before/A.hs

@@ -0,0 +1,9 @@
+module A where
+
+data Box t = SomeBox t
+
+instance Functor Box where
+    fmap f (SomeBox a) = SomeBox (f a)
+
+instance Applicative Box where
+    pure a = SomeBox a

message-index/messages/GHC-06201/example2/index.md

@@ -0,0 +1,26 @@
+---
+title: No explicit implementation for method instance 2
+---
+
+## Error Message
+
+```
+A.hs:8:10: warning: [GHC-06201] [-Wmissing-methods]
+    • No explicit implementation for
+        either ‘<*>’ or ‘liftA2’
+    • In the instance declaration for ‘Applicative Box’
+  |
+8 | instance Applicative Box where
+  |          ^^^^^^^^^^^^^^^
+```
+
+## Explanation
+
+The type class `Applicative` requires you to implement at least two methods:
+
+* `pure`, and
+* `liftA2` or `multiply` (can also be both).
+
+Such `or` constraints can be expressed via the [`{-# MINIMAL #-}`](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/pragmas.html#minimal-pragma) pragma.
+
+To fix this, either implement the method `liftA2` or `<*>`. For this example, we implemented `<*>`, but either or both is fine, too.

message-index/messages/GHC-06201/index.md

@@ -0,0 +1,9 @@
+---
+title: Missing method in type class instance
+summary: A required method is missing from the instance declaration.
+severity: warning
+flag: -Wmissing-methods
+introduced: 9.6.1
+---
+
+This warning means that a type class instance is missing some required method implementations.