Add GHC-67120 main not defined (#555)

Author: noughtmare

message-index/messages/GHC-67120/example1/after/Lib.hs

@@ -0,0 +1,4 @@
+module Lib where
+
+factorial :: Int -> Int
+factorial n = product [1..n]

message-index/messages/GHC-67120/example1/before/Lib.hs

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+factorial :: Int -> Int
+factorial n = product [1..n]

message-index/messages/GHC-67120/example1/index.md

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+---
+title: Missing module header
+---
+
+In this example, the file `Lib.hs` is intended to contain a module called `Lib`
+which is not supposed to be the main module. However, the file is missing a
+module header, so GHC defaults to the module name `Main` and expects the `main`
+function. Adding an explicit module header with the name `Lib` solves this
+issue.

message-index/messages/GHC-67120/example2/after/Main.hs

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+main :: IO ()
+main = putStrLn "Hello, World!"

message-index/messages/GHC-67120/example2/before/Main.hs

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+notMain :: IO ()
+notMain = putStrLn "Hello, World!"

message-index/messages/GHC-67120/example2/index.md

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+---
+title: main has wrong name
+---
+
+In this example, the `notMain` value is the intended entry point of the program,
+but it is not called `main` so GHC does not find it. Renaming `notMain` to 
+`main` solves the issue.

message-index/messages/GHC-67120/index.md

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+---
+title: Missing main
+summary: The IO action ‘main’ is not defined in module ‘Main’.
+introduced: 9.8.1
+severity: error
+---
+
+GHC expects the `Main` module to define a function called `main` which it can
+use as the entry point of your program.
+
+This error can also occur in unnamed modules, because GHC will default to the
+module name `Main` for such modules.
+
+If you just want GHC to produce an object file without an entry point, then you
+can give your module a name other than `Main` by putting a module header at the
+top of your file (below language pragmas and compiler options), for example as
+follows:
+
+```
+module Foo where
+```
+
+The conventions around `main` are defined in the second paragraph of Chapter 5
+of [The Haskell 2010 Report](https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-980005),
+
+> A Haskell program is a collection of modules, one of which, by convention,
+> must be called `Main` and must export the value `main`. The value of the
+> program is the value of the identifier `main` in module `Main`, which must be
+> a computation of type `IO t` for some type `t` (see Chapter 7). When the
+> program is executed, the computation `main` is performed, and its result (of
+> type `t`) is discarded.
+
+## Example error text
+
+```
+example1/before/Lib.hs:1:1: error: [GHC-67120]
+    The IO action ‘main’ is not defined in module ‘Main’
+  |
+1 | factorial :: Int -> Int
+  | ^
+```
+
+```
+example2/before/Main.hs:1:1: error: [GHC-67120]
+    The IO action ‘main’ is not defined in module ‘Main’
+  |
+1 | notMain :: IO ()
+  | ^
+```