Cannot use DefaultSignatures for TypeFamiles (#16027) · Issues · Glasgow Haskell Compiler / GHC

Cannot use DefaultSignatures for TypeFamiles

I'm surprised there wasn't a ticket for this. Maybe I just couldn't find it.

In short, I want to do something like:

class Monad m => MyMonad m where
    type AssociatedMonad m :: Type -> Type
    default type AssociatedMonad m :: (m ~ t m', MonadTrans t, MyMonad m') => Type -> Type
    type AssociatedMonad (t m') = t (AssociatedMonad m')

    fun :: Foo -> m ()
    default fun :: (m ~ t m', MonadTrans t, MyMonad m') => Foo -> m ()
    fun = lift . fun

The syntax is a bit weird, but I hope in the process of implementing constrained type families the syntax properly analogous to the term-level would shake itself out.

The use-case, like the normal method original, is to assist with monad transformer boilerplate. MonadTrans wouldn't be enough, but MFunctor from mmorph might be depending on the method using the associated type.

Trac metadata

Trac field	Value
Version	8.6.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

I'm surprised there wasn't a ticket for this. Maybe I just couldn't find it.

In short, I want to do something like:

```
class Monad m => MyMonad m where
    type AssociatedMonad m :: Type -> Type
    default type AssociatedMonad m :: (m ~ t m', MonadTrans t, MyMonad m') => Type -> Type
    type AssociatedMonad (t m') = t (AssociatedMonad m')

fun :: Foo -> m ()
    default fun :: (m ~ t m', MonadTrans t, MyMonad m') => Foo -> m ()
    fun = lift . fun
```

The syntax is a bit weird, but I hope in the process of implementing constrained type families the syntax properly analogous to the term-level would shake itself out.

The use-case, like the normal method original, is to assist with monad transformer boilerplate. `MonadTrans` wouldn't be enough, but `MFunctor` from mmorph might be depending on the method using the associated type.

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

| Trac field             | Value          |
| ---------------------- | -------------- |
| Version                | 8.6.2          |
| Type                   | FeatureRequest |
| TypeOfFailure          | OtherFailure   |
| Priority               | normal         |
| Resolution             | Unresolved     |
| Component              | Compiler       |
| Test case              |                |
| Differential revisions |                |
| BlockedBy              |                |
| Related                |                |
| Blocking               |                |
| CC                     |                |
| Operating system       |                |
| Architecture           |                |

</details>