[Haskell-beginners] how do typeclasses work again?

[Nicholls, Mark]

I'm naively capable of messing around with type families...so I know how to define the types statically...that's not really what I want...that's too strong.

I think I'm trying to work in the universe of typeclasses and not data types....in my OO head these two things overlap (if you see what I mean)....in Haskell they are distinct...which I'm beginning to feel makes type inference easy....but is actually quite "weak".

so lets start again...

> class Is isx x where
>   apply ::(x -> y) -> isx -> y

> instance Is x x where
>   apply f = f

i.e. lets create our tuple instance like this!

> instance (Is m x) => Is (m,y) x where
>   apply f (m,y) = apply f m 

> data X = X deriving (Show)
> data Y = Y deriving (Show)

> foo4 :: forall a isx. (Is isx a, Show a) => isx -> String
> foo4 = apply (\(i :: a) -> show i)

now...this line said....

> main = print (foo4 @X (X,Y))

and that works!....which I think is what I want....in an OO world this feels like a "cast"....where Ive said (X,Y) <: X....I'm getting the compiler to extract fst for me...I'm lazy.

so lets tell the compiler it could do snd for me.

> instance (Is m x) => Is (y,m) x where
>   apply f (y,m) = apply f m 

gives....."Duplicate instance declarations "

which is unfortunate as I wanted to then write

> main = print (foo4 @Y (X,Y))

to "cast" to Y....which feels perfectly reasonable

then I look this up on the interweb...and magically found some noob has been here before!
"noob “Duplicate instance declarations” (again)"

that noob was me!...about a year ago...and someone said I was misunderstanding how to define these sort of recursive structures....and It should be done in the class declaration...which doesn't seem to work in this case...as I want to do something recursive 

the answer said.....

"Haskell requires that there be only one instance for each class and type. Thus is determined only from the part to the right of the =>. "

ok, I buy that...IF I want to guarantee Haskell to automatically derive type class dictionaries that functional restriction is perfectly reasonable...as long as its resolved at some point.

I tbink (naively)...Haskell is saying...
"if I match things against (x,y) I've got 2 instance declarations....so how do I decide which one?"

what I'm saying is....that's fine but I'm telling you which one at the call site....using these "@" things...

so what's the problem?...the functional restriction is too restrictive.












________________________________________
From: Beginners [beginners-bounces at haskell.org] on behalf of Sylvain Henry [sylvain at haskus.fr]
Sent: 11 February 2017 13:36
To: beginners at haskell.org
Subject: Re: [Haskell-beginners] how do typeclasses work again?

On 11/02/2017 09:50, Nicholls, Mark wrote:
> I don't understand the difference
>
> forall a isx. (Is isx a, Show a) => isx -> String
> and
> (Is isx a, Show a) => isx -> String
I have used the `forall` explicitly only to fix the order of the type
parameters (`a` and `isx`) so that we are sure to set the type of `a`
when we write (using TypeApplications): foo4 @Y (X,Y)

In the second declaration, the `forall` is implicit.

> I think my understanding of type classes is naïve, I just thought it meant that secretly a dictionary was being passed.
Yes your understanding is correct. The issue here is that the compiler
doesn't know the type of `a`, hence it can't select and pass the
appropriate instances.

> the compiler would identify the specific dictionary from the call site
Even at call site, the compiler can't infer the `a` type from the `isx`
type (nor from the return type of `foo4`).

Do you want the `a` type to be dependent on the `isx` type? I.e., to
only be allowed to define a single `Is isx a` instance for each `isx` type.

--
Sylvain
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