Chris Smith wrote,
ChrisK wrote:
-- You need to help GHC more here:
type instance A_T () = <explicit type you actually use with ()>
type instance A_T (Var x) = <explicit type you actually use with (Var x)>
type instance A_T [t] = t
type instance A_T (RHS (Var x) a) = <explicit type you actually use ...>
type instance A_T (RHS [t] a) = t
Chris, this actually isn't workable. The problem is that there is no
such thing as "explicit type you actually ue with ()". If the type
variable 'a' gets matched to () or (Var x), then t can be any type.
That works currently. I don't want to fix the type of 't', as this code
goes in a library where different applications are using different types
for 't'.
As I said, this is probably not really right, since () really doesn't
determine a value for a; but it does work right now.
Just for kicks, I tried this:
type instance A_T () = forall t. t
it didn't work.
Right-hand sides of type instances may not be polytypes. There is a
counter example in Section 7.2,
http://haskell.org/haskellwiki/GHC/Type_families
but the text didn't explicitly make that point. I edited it to
include that.
Cheers,
Manuel
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