Type constraints on all type family instances

Question

I suppose what I want is impossible without Template Haskell but I'll ask anyway. I have an interface for types like `Data.Set` and `Data.IntSet`: ``` type family Elem s :: * class SetLike s where insert :: Elem s -> s -> s member :: Elem s -> s -> Bool ... type instance Elem (Set a) = a instance Ord a => SetLike (Set a) where ... ``` And I have a type family which chooses optimal set implementation: ``` type family EfficientSet elem :: * type instance EfficientSet Int = IntSet type instance EfficientSet String = Set String -- or another implementation ``` Is there a way to guarantee that `EfficientSet` instances will be always `SetLike` and that `Elem (EfficientSet a)` is `a` ? Without this guarantee all function signatures will be like this: ``` type LocationSet = EfficientSet Location f :: (SetLike LocationSet, Elem LocationSet ~ Location) => ... ``` To write each time `SetLike LocationSet` is somewhat tolerable, but `Elem LocationSet ~ Location` makes code understanding only harde…

Accepted Answer

Using GHC 7.4's constraint kinds you could have something like

```
type EfficientSetLike a = (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a)
```

You can (with appropriate extensions) get constraints like this in earlier versions of GHC

```
class (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 
instance (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 
```

But, the new style `type` declaration is much nicer.

I'm not exactly sure what you are looking for, but it sounds like you just want easier to write/understand constraint signatures, in which case this will work.

Type constraints on all type family instances

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