Type families in kind signatures with TypeInType

[Simon Peyton Jones]

Interesting. Is this case also an example, or is it a non-feature?

class C t where
    type K t :: Type
    type T t :: K t -> Type

    m :: t -> T t a


Ah, that’s quite different!  We should do strongly-connected-component analysis of the associated-type declarations within a single class declaration…. but we don’t currently do that.   No difficulty in principle, I think.

You could open a ticket.   (Do include a link to this email thread and to #12088)

Simon


From: d4ve.menendez at gmail.com [mailto:d4ve.menendez at gmail.com] On Behalf Of David Menendez
Sent: 23 September 2016 19:51
To: Simon Peyton Jones <simonpj at microsoft.com>
Cc: glasgow-haskell-users at haskell.org Mailing List <Glasgow-haskell-users at haskell.org>
Subject: Re: Type families in kind signatures with TypeInType

On Fri, Sep 23, 2016 at 3:19 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
This is an example of https://ghc.haskell.org/trac/ghc/ticket/12088.

Interesting. Is this case also an example, or is it a non-feature?

class C t where
    type K t :: Type
    type T t :: K t -> Type

    m :: t -> T t a

min.hs:21:17: error:
    • Type constructor ‘K’ cannot be used here
        (it is defined and used in the same recursive group)
    • In the kind ‘K t -> Type’
Failed, modules loaded: none.

GHC accepts this if K t is moved outside of C.

The “type instance T List” declaration actually depends on the “type instance K List” declaration; the latter must be typechecked before the former.  But this dependency is absolutely unclear.  There’s a long discussion on the thread.  Bottom line: we don’t know a solid automated way to spot this kind of problem, so  I think we are going to ask for programmer assistance.  In this case, we’d put a “separator” after the “type instance K List” decl, to explain that it must be done first:

    type instance K List = Type
    ===========
    type instance T List = []

Currently you have to write $(return []) to get the separator, but I think we’ll add a special separator.

Yes, this works. Thanks.

It would be disappointing if this is the best we can do, but I guess other dependent languages don’t need to deal with open type families and everything being potentially mutually recursive.

--
Dave Menendez <dave at zednenem.com<mailto:dave at zednenem.com>>
<http://www.eyrie.org/~zednenem/<https://na01.safelinks.protection.outlook.com/?url=http:%2F%2Fwww.eyrie.org%2F~zednenem%2F&data=01%7C01%7Csimonpj%40microsoft.com%7C89f26f5e599e49b96e0c08d3e3e29e26%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=c8g6ahf%2F8lun%2BPBof97s03XbePVwT4Buh6ef2bDSjLg%3D&reserved=0>>
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