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Today's Topics:
1. Re: how do typeclasses work again? (Sylvain Henry)
2. Re: how do typeclasses work again? (Nicholls, Mark)
----------------------------------------------------------------------
Message: 1
Date: Sat, 11 Feb 2017 14:36:36 +0100
From: Sylvain Henry <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] how do typeclasses work again?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
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
------------------------------
Message: 2
Date: Sun, 12 Feb 2017 10:05:53 +0000
From: "Nicholls, Mark" <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: Re: [Haskell-beginners] how do typeclasses work again?
Message-ID:
<e7e7fdf32472ff48bb0e4d9dc4283d0e8bd09...@mtvne-exmb02.mtvne.ad.viacom.com>
Content-Type: text/plain; charset="windows-1252"
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 [[email protected]] on behalf of Sylvain Henry
[[email protected]]
Sent: 11 February 2017 13:36
To: [email protected]
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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