[Haskell-cafe] Re: Are type synonym families really equivalent
Manuel M T Chakravarty
chak at cse.unsw.edu.au
Tue Sep 4 01:06:59 EDT 2007
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,
but the text didn't explicitly make that point. I edited it to
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