It's not that your syntax is off, but rather that what you're trying to do doesn't really make sense in the Haskell type system. Lists are by nature parameterized over a single type. It is possible via existential types to have a list over a polymorphic type like: exists a. (Locatable a, Animation a) => a but this is doubtfully what you want. It suffers from the fact that, for a given list entry, you don't actually know what its type is, and so you can't do anything really useful here. So basically: you can't have two kinds of data types in a list. What you probably want is a data type, like: data Object = Star ... | Asteroid ... That is, rather than having two types that you're trying to put in a list, have one type with two data constructors. Then you *can* store them together, and write functions that operate via pattern matching on the various constructors. Hope that helps. Nick On Friday, August 31, 2012 11:34:40 AM Christopher Howard wrote:
Hi. I've got two data structures, Star and Asteroid (eventually I'll make more) that both belong to the following type classes: Locatable, and Animation. I wanted to do something like so in part of the code:
code: -------- let stars = ... in -- of type [Star] let asteroids = ... in -- of type [Asteroid] let visibleObjects = do visibleObject <- (stars ++ asteroids) ... -- prep each object for graphics system -- using funcs from both type classes ... -- feed visibleObjects to graphics system --------
However, this does not work because the two lists are not automatically downgraded when joined together by (++). The compiler complains about "asteroids" not being of type [Star]. What is the simplest way to do what I am trying to do? I tried this, but I think my syntax is off:
code: -------- let visibleObjects = do visibleObject <- ((stars :: [(Locatable a, Animation a) => a) ++ (asteroids :: [(Locatable a, Animation a) => a) ) --------
Compiler complains about "Illegal polymorphic or qualified type".