GHC 7.10 rejects nullary type class with associated data (#10020) · Issues · Glasgow Haskell Compiler / GHC

GHC 7.10 rejects nullary type class with associated data

In GHC 7.8.4, the following code is legal:

{-# LANGUAGE NullaryTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData

In GHC 7.10, NullaryTypeClasses was deprecated in favor of MultiParamTypeClasses. However, running this code on GHC 7.10-rc1:

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData

results in this error:

NullaryData.hs:4:1:
    The associated type ‘NullaryData’
    mentions none of the type or kind variables of the class ‘NullaryClass’
    In the class declaration for ‘NullaryClass’

It's probably related to this earlier bugfix.

Trac metadata

Trac field	Value
Version	7.10.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

In GHC 7.8.4, the following code is legal:

```hs
{-# LANGUAGE NullaryTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData
```

In GHC 7.10, `NullaryTypeClasses` was [deprecated](https://ghc.haskell.org/trac/ghc/ticket/8993) in favor of `MultiParamTypeClasses`. However, running this code on GHC 7.10-rc1:

```hs
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
module NullaryData where

class NullaryClass where
    data NullaryData
```

results in this error:

```
NullaryData.hs:4:1:
    The associated type ‘NullaryData’
    mentions none of the type or kind variables of the class ‘NullaryClass’
    In the class declaration for ‘NullaryClass’
```

It's probably related to [this earlier bugfix](https://ghc.haskell.org/trac/ghc/ticket/9167).

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 7.10.1-rc1   |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>