[Haskell-cafe] Problems with type families
Leza Morais Lutonda
leza.ml at fecrd.cujae.edu.cu
Thu Jul 9 01:12:07 UTC 2015
On 07/06/2015 07:29 PM, Silvio Frischknecht wrote:
> looks like the problem are the 'e's. you can see it from
> Could not deduce (TFunOut t1 e ~ TFunOut t1 e0)
> the e0 comes from the output of TFunIn/TFunOut and the instance does not
> know that it is the same as the e variable you use.
> I don't know exactly what your program does. but if e is just a
> phantom/security parameter you can do it like this.
> type family TFunIn t :: * -> *
> type instance TFunIn IdT = Signal
> type instance TFunIn (ComposeT t1 t2) = TFunIn t2
> instance (SignalT t1 e, SignalT t2 e, TFunOut t2 ~ TFunIn t1)
> => SignalT (ComposeT t1 t2) e where
> transform _ x = (transform (undefined :: t1) .
> transform (undefined :: t2)) x
> Otherwise you will have to find a way to prove that the 'e's are the same
> Cheers Silvio
The `e` type is important. The problem with not including `e` in the
`TFunIn` is that I will want to define:
type instance TFunIn DoubleT e = (Signal e, Signal e)
which I cannot express without specifying the `e` type parameter.
And it cannot be a closed type family. So what can I prove that the `e`s
are the same?
The intuition I get from the instance declaration is that "they/it" are
instance (SignalT t1 e, SignalT t2 e, TFunOut t2 e ~ TFunIn t1 e) => SignalT (ComposeT t1 t2) e
transform _ x = (transform (undefined :: t1) . transform (undefined :: t2)) x
Leza Morais Lutonda, Lemol-C
50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu
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