Hi Erik, Thanks for the tip. Interestingly, on Reddit [1] just appeared almost the same problem with some suggestions. vlatko [1] - http://www.reddit.com/r/haskell/comments/2jsz4t/function_on_a_single_member_... -------- Original Message -------- Subject: Re: [Haskell-cafe] Limit elements of a list to a certain value From: Erik Hesselink To: Vlatko Bašić Cc: haskell Date: 20.10.2014 11:55

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 wrote:

...

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