Type synonyms with hidden, determined type variables

I found myself wanting to write (simplified)

type MyMonad m = (MonadReader r m, Show r)

but GHC does not accept this, it wants r as part of the type synonym. So why did I think this could work? Because MonadReader has a functional dependency:

class Monad m => MonadReader r m | m -> r

so if m determines r, why do I have to write MyMonad r m and not just MyMonad m? Can I not “hide” the fact that, somewhere internal to the MyMonad synonym, there is an additional variable r around?

Clearly supporting this would need a language proposal, but I wanted to open a ticket here first to hear if this would even be possible to implement.

(Others have wanted this before as well.)

Trac metadata

Trac field	Value
Version	8.5
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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