On Thu, 28 Jul 2022, Simon Peyton Jones wrote:
Wild guess: you aren't instantiating the kinds of the meta-tyvars to something
sensible, so kinds don't line up. Eg. where are k_ax9 and k_awW bound?
`a_auu :: k_ax9` is the result of typechecking my type argument (the `a`
in the source type `MySyn a`), so I would expect its kind to work because
it is computed by parts of GHC that I am not changing.
`w_awX :: k_awW` is the metavariable created during the typechecking of
the right-hand side of my type macro `type MySyn a = MyData a _ Int`. This
kind is then kept for the fresh metavariable that I try to replace `w_awX`
with, since I have `w_axc :: k_awW`.
The part that confuses me the most is that the right-hand side seems to
work just fine on its own:
As an experiment, I have also tried *not* refreshing `w_awX`, only
substituting in the type arguments. Now, of course, this can't possibly
work as soon as I have more than one occurrence of `MySyn` due to the
interference between the wildcard metavars, but if I only have one, then
the program typechecks.
So I have two versions of my code returning two types from
`tc_infer_hs_type` that only differ in containing either `w_awX :: k_awW`
or `w_axc :: k_awW`, and one of them typechecks while the other causes a
type error.
Typechecks (as long as only used once of course):
```
TySynWildcard.MyData{tc r3}
(a{tv auu} Nothing [sk:2] :: (k_ax9{tv} Nothing [tau:2] ::
GHC.Types.Type{(w) tc 32Q}))
((w_awX{tv} Nothing [tau:0] :: (k_awW{tv} Nothing [tau:0] ::
GHC.Types.Type{(w) tc 32Q}))
|> {(co_awY{v} Just 'GHC.Types.Many{(w) d 65I} [lid[CoVarId]] ::
GHC.Prim.~#{(w) tc 31I}
GHC.Types.Type{(w) tc 32Q}
GHC.Types.Type{(w) tc 32Q}
(k_awW{tv} Nothing [tau:0] :: GHC.Types.Type{(w) tc 32Q})
GHC.Types.Type{(w) tc 32Q})})
GHC.Types.Int{(w) tc 3u}
```
Doesn't typecheck:
```
TySynWildcard.MyData{tc r3}
(a{tv auu} Nothing [sk:2] :: (k_ax9{tv} Nothing [tau:2] ::
GHC.Types.Type{(w) tc 32Q}))
((w_axc{tv} Nothing [tau:2] :: (k_awW{tv} Nothing [tau:0] ::
GHC.Types.Type{(w) tc 32Q}))
|> {(co_awY{v} Just 'GHC.Types.Many{(w) d 65I} [lid[CoVarId]] ::
GHC.Prim.~#{(w) tc 31I}
GHC.Types.Type{(w) tc 32Q}
GHC.Types.Type{(w) tc 32Q}
(k_awW{tv} Nothing [tau:0] :: GHC.Types.Type{(w) tc 32Q})
GHC.Types.Type{(w) tc 32Q})})
GHC.Types.Int{(w) tc 3u}
```
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