[Haskell-beginners] How do I give a type for this code?

[Daniel Trstenjak]

Hi Todd,

On Thu, Oct 17, 2013 at 09:58:08PM -0700, Todd Wilson wrote:
> Thanks, Daniel, for your suggestion, but I think it misses a key aspect of
> my situation (unless I've misunderstood).  Every model is supposed to be a
> quadruple (m, q, f, p), with m a list, q an element of m, f a list of
> ordered pairs of elements of m, and p a sublist of m.  By contrast, it
> looks like your definition makes a model either a single element, or a
> single pair, or a single list, etc.
> --Todd

Sorry, I've been rushing a bit.


type Model a = ([a], a, [(a,a)], [a])  -- a is the "base type"

The main problem is, that you can't use different base types for your Model at once.
If you want to support '()', 'a', '(a,a)' and '[a]', than you need one type to abstract
over them.

data BaseType a = Unit             -- ()
                | Single a         -- a
                | Product (a,a)    -- (a,a)
                | Power [a]        -- [a]


And now you define your Model by using the BaseType:

type Model' a = Model (BaseType a)


Greetings,
Daniel