[Haskell-beginners] newbie's perplexities about instance of "Info a" of Chapter 13 (Thompson's Haskell book)

[David McBride]

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 at 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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