Confused about specified type variables using -XTypeApplications

[Ryan Scott]

I've opened Trac #11376 for this: https://ghc.haskell.org/trac/ghc/ticket/11376

Ryan S.

On Thu, Jan 7, 2016 at 5:32 PM, Simon Peyton Jones
<simonpj at microsoft.com> wrote:
> Make a ticket?  This would be worth capturing.
>
> I'm sure Richard will want to look at it when he gets back, and a ticket is a useful workflow organiser.
>
> Simon
>
> | -----Original Message-----
> | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ryan Scott
> | Sent: 07 January 2016 17:55
> | To: GHC developers <ghc-devs at haskell.org>
> | Subject: Confused about specified type variables using -XTypeApplications
> |
> | I read in the Visible Type Applications paper [1] that you can only
> | apply types to "specified" type variables. However, after trying out
> | -XTypeApplications, I'm confused as to what exactly that means:
> |
> |     $ /opt/ghc/head/bin/ghci
> |     GHCi, version 8.1.20160106:
> | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fwww.haskell.o
> | rg%2fghc%2f&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7caa1fba8c2743424
> | c1fb008d3178bb622%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=2B1fES1XwoQb8T
> | MRj7oOkfdfoGo37oXhjnHFeqgOXAI%3d  :? for help
> |     λ> :set -XTypeInType -XTypeApplications
> |     λ> data Prox a = Prox
> |     λ> let prox :: Prox a; prox = Prox
> |     λ> :t prox @Int
> |     prox @Int :: Prox Int
> |     λ> :t Prox @Int
> |     Prox @Int :: Prox a
> |
> | Huh? For some reason, I'm getting different types for prox @Int and
> | Prox @Int! I think what's happening here is that in Prox @Int, the Int
> | is being applied to a kind variable. That is, it's being applied to
> | the k variable in:
> |
> |     Prox :: forall k (t :: k). Prox t
> |
> | But why is this? After all, I don't think I "specified" k anywhere in
> | the definition of Prox, and to make things more confusing, Int gets
> | applied differently in prox @Int (and the definition of prox also
> | doesn't mention k). Is this a bug, or am I misunderstanding something
> | about -XTypeApplications?
> |
> | Ryan S.
> | -----
> | [1]
> | https://na01.safelinks.protection.outlook.com/?url=http:%2f%2fwww.cis.upenn.e
> | du%2f~eir%2fpapers%2f2016%2ftype-app%2fvisible-type-
> | app.pdf&data=01%7C01%7Csimonpj%40064d.mgd.microsoft.com%7Caa1fba8c2743424c1fb
> | 008d3178bb622%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=PKUXsuMdixF76cVq0y
> | yaCW8NZbIO5hOJpXwABfUkYZw%3d
> | _______________________________________________
> | ghc-devs mailing list
> | ghc-devs at haskell.org
> | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.
> | org%2fcgi-bin%2fmailman%2flistinfo%2fghc-
> | devs%0a&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7caa1fba8c2743424c1fb
> | 008d3178bb622%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=hV%2f3yHwiH36Qgegu
> | hepSlx7JaZP0evwpGTTdK%2f3I3Po%3d