This is an example of https://ghc.haskell.org/trac/ghc/ticket/12088.
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.
Simon
From: Glasgow-haskell-users [mailto:[email protected]]
On Behalf Of David Menendez
Sent: 23 September 2016 05:48
To: [email protected] Mailing List
<[email protected]>
Subject: Type families in kind signatures with TypeInType
Should the code below work in GHC 8.0.1?
{-# LANGUAGE TypeInType, TypeFamilies #-}
import Data.Kind (Type)
type family K t :: Type
type family T t :: K t -> Type
data List
type instance K List = Type
type instance T List = []
Right now, I get an error like this one:
min.hs:12:24: error:
• Expected kind ‘K List -> Type’, but ‘[]’ has kind ‘* -> *’
• In the type ‘[]’
In the type instance declaration for ‘T’
which is puzzling, since K List -> Type and * -> * should be the same.
Obviously, TypeInType is experimental and incomplete. I’m just wondering if
this sort of thing is expected to work, or if I’m doing something not yet
supported or never to be supported
In particular, the kind signature for T is forall t -> K t -> Type, which looks
like DependentHaskell.
--
Dave Menendez <[email protected]<mailto:[email protected]>>
<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%7C740be9e2f66146e93b1308d3e36cdcda%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=r83909O38f2c%2feaujupnsW%2fSy23XLouk2unQBGTCB8w%3d>>
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