Document error GHC-05380

Explain why a newtype only can have 1 constructor.

Author: jorisburgers

message-index/messages/GHC-05380/index.md

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+---
+title: Multiple newtype constructors
+summary: More than 1 constructor for a newtype
+severity: error
+introduced: 9.6.1
+---
+
+A newtype constructor can only have a single constructor. If you want multiple
+constructors, you should use `data` instead of `newtype`.
+
+The reason that you need exactly one constructor is that `newtype` constructors
+are removed at runtime.
+So during runtime, if you define `newtype Age = Age Int`, `Age` is the same as `Int`.
+Only during the compilation phase, will the `Age` and `Int` types be different.
+If we allow multiple constructors for a newtype, there would be no way to distinguish them during execution.

message-index/messages/GHC-05380/more-than-1-constructor/after/More-than-1-constructor.hs

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+data Direction = Up | Down

message-index/messages/GHC-05380/more-than-1-constructor/before/More-than-1-constructor.hs

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+newtype Direction = Up | Down

message-index/messages/GHC-05380/more-than-1-constructor/index.md

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+---
+title: More than 1 constructor for a newtype
+---
+
+## Error Message
+```
+Main.hs:1:1: error: [GHC-05380]
+    A newtype must have exactly one constructor,
+      but ‘Direction’ has two
+    In the newtype declaration for ‘Direction’
+  |
+1 | newtype Direction = Up | Down
+  |
+```
+## Explanation
+
+A newtype constructor can only have a single constructor. It is not allowed to have multiple
+constructors when defining a newtype.

message-index/messages/GHC-05380/zero-constructors/after/Zero-constructors.hs

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+data Void

message-index/messages/GHC-05380/zero-constructors/before/Zero-constructors.hs

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+newtype Void

message-index/messages/GHC-05380/zero-constructors/index.md

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+---
+title: Zero constructors for a newtype
+---
+
+## Error Message
+```
+Main.hs:1:1: error: [GHC-05380]
+    A newtype must have exactly one constructor, but ‘Void’ has none
+    In the newtype declaration for ‘Void’
+  |
+1 | newtype Void
+  |
+```
+
+## Explanation
+
+A `newtype` constructor does need to have a single constructor. It is not allowed to
+have a newtype without a constructor. It is allowed to have a `data` type without a
+constructor, so in order to fix this, you need to come up with a constructor for the newtype
+or change the `newtype` keyword to a `data` keyword.