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Basic explanation of 59692 #534

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Basic explanation of 59692 #534

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6 changes: 6 additions & 0 deletions message-index/messages/GHC-59692/example1/after/Example1.hs
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module Example1 where

data T a

instance Eq a => Eq (T a) where
(==) = (==)
9 changes: 9 additions & 0 deletions message-index/messages/GHC-59692/example1/before/Example1.hs
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module Example1 where

data T a

instance Eq a => Eq (T a) where
(==) = (==)

instance Show a => Eq (T a) where
(==) = (==)
15 changes: 15 additions & 0 deletions message-index/messages/GHC-59692/example1/index.md
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Even though the instances have different constraints, they are still considered the same.

## Error message

```
messages/GHC-59692/example1/before/Example1.hs:3:10: error: [GHC-59692]
Duplicate instance declarations:
instance Eq a => Eq (T a)
-- Defined at messages/GHC-59692/example1/before/Example1.hs:3:10
instance Show a => Eq (T a)
-- Defined at messages/GHC-59692/example1/before/Example1.hs:6:10
|
3 | instance Eq a => Eq (T a) where
| ^^^^^^^^^^^^^^^^
```
10 changes: 10 additions & 0 deletions message-index/messages/GHC-59692/index.md
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---
title: Duplicate instance declarations
summary: An instance has been defined twice for the same type.
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It's not necessarily exactly twice, is it? A more precise way to describe this would be, perhaps:

Suggested change
summary: An instance has been defined twice for the same type.
summary: An instance of a class has been defined once again for the same type.

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"once again" makes it sound like there was an instance in the past and now there is one once again. Perhaps just "multiple times" instead of "twice" then.

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But for the compiler, there was an instance that it saw in the past, only it's very close past: maybe in the same .hs file (but maybe not). So, I don't see a problem here. But "multiple times" works for me too.

introduced: 9.6.1
severity: error
---

You can only define a single instance for every type, because it must be possible to automatically resolve class constraints without any ambiguity.
Note that instances are considered the same even if they have different constraints. Only the arguments of the class are considered when matching instances.
See the [GHC User Guide](https://downloads.haskell.org/~ghc/9.10.1/docs/users_guide/exts/instances.html) for more information on instance declarations and resolution.
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