[Haskell-beginners] joining lists sharing multiple type classes

[Nick Vanderweit]

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".