You could do it by making Parsed into a GADT:
data Parsed ev where
PEvent :: Event -> Parsed E1
...
As for the question about promoting 'Q', I think it's because it
contains a String, and the Symbol kind doesn't map one-to-one to
String, since it's opaque. The same goes for types containing
Integers, I guess. They also cannot be promoted, as Integer doesn't
map cleanly to Nat. I'm just guessing here, though.
Erik
On Mon, Oct 20, 2014 at 11:02 AM, Vlatko Basic
Hello Cafe,
I have these datatypes
data Event = E1 {...} | E2 {...} | E3 {...} data Parsed = PEvent Event | PLine Text | PHeader Text
I'd like to split the list of Parsed values into separate lists with a guaranty that each of those lists contains only the elements of the same type, as in [PEvent E1] [PEvent E2] [PLine Text] ...
How can that be achieved on type level (without creating newtypes for each case)? I tried with type families, but failed.
Also, an explanation of why the Event above (or "data Q = Q1 | Q2 String") is not promotable to kind level would be welcome.
best regards,
vlatko _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe