In ghci try this:

> examples :: [Int]

It is enough that a is mentioned in the type of every function so that the compiler can figure out which instance is required when it is used. In size you have to pass an a in to figure out what type it is, but in examples, it is enough to know that it is a list of a's.

On Tue, Sep 10, 2013 at 1:59 PM, Carmine Moleti <carmine.moleti@gmail.com> wrote:

Hi to everyone,

I'm learning Haskell going through Thompson's craft of functional
programming (3rd ed).

As of now I've reached Chapter 13 where classes definition and instances are
discussed.

At some point the "Info a" class is introduced which has the following
interface:

class Info a where
examples :: [a]
size :: a -> Int

then it goes on explaining that every instance of that class must comply to
the interface shown. So far so good.

My perplexities start when actual instances are implemented.

Take, for example the following one:

instance Info Int where
examples = [-100..100]
size _ = 1

I promptly tried to fiddle with this via ghci and while I was able to access
the "size" function of the interface I have no clue about how to reach the
"examples" part and that prevents me from understanding the next
step when an instance of "Info [a]" is discussed.
About that I don't quite get the definition of its "examples", and the
reasons it is defined taking into accounts "all the one and two element
lists that can be built up from exmaples of type a" (cit.)

Can anyone shed some light about this, please?

Thanks in advance for your help.

Best regards,
Carmine

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