[Haskell-beginners] Alternative instance w/ additional restriction

[David McBride]

I honestly don't think so.  But you can ask on haskell-cafe or on
stackoverflow if you like.  There is a reasonable chance that a class
or library exists that already will do what you are looking for, just
that I haven't heard of it.

On Wed, Jun 7, 2017 at 12:06 PM, Baa <aquagnu at gmail.com> wrote:
> So, if I understood right, I'm trying to restrict instance more than
> class allows. And only way to accomplish it - is to create own class
> instead of Alternative which will not be based on
> Functor :: (* -> *) -> *, like Alternative. No other workarounds?
>
>
>
> В Wed, 7 Jun 2017 11:25:26 -0400
> David McBride <toad3k at gmail.com> пишет:
>
>> The Alternative class says nothing about the a in MyData a.  It only
>> represents code relevant to MyData.
>>
>> When you see class Applicative f => Alternative (f :: * -> *), that
>> means all of its functions had to work on any f, where f takes any
>> type and becomes some other type which could be anything.
>>
>> The reason it works for Monoid is that class Monoid a where implies
>> that a is completely known by the time the instance is fulfilled,
>> therefore the instance can look at what a ended up being and ensure
>> whatever a is, it must have this constraint on it.
>>
>> You can tell the difference because mempty returns a type that
>> mentions the a mentioned in the class, whereas empty returns an a that
>> is not mentioned in the class, therefore it has to work for any a.
>>
>> On Wed, Jun 7, 2017 at 8:26 AM, Baa <aquagnu at gmail.com> wrote:
>> > Hello all!
>> >
>> > If I try to write, for example:
>> >
>> >   instance Alternative MyData where
>> >     empty = NoMyData
>> >     a <|> b = if a == b then ...
>> >
>> > I get error (see on bottom of the mail) sure, bcz I suppose
>> > something like:
>> >
>> >    Eq a => (MyData a)
>> >
>> > All my attempts to add something to instance's `pure` signature have
>> > failed. How can I instantiate something with additional
>> > restrictions, like in this case? Or are there another solutions for
>> > such problem?
>> >
>> > Interesting is that MyData derives Eq itself! Which, I suppose, must
>> > means that "Eq (MyData a)", and if it's true than "Eq a" is true,
>> > because how "MyData a" can be Eq without to be "Eq a" (with
>> > *automatically deriving* of Eq instance) ?!
>> >
>> > By the way, for Monoid (which is "* -> Constraint") I can add "Eq a"
>> > constraint without problems:
>> >
>> >   instance Eq a => Monoid (Origin a) where
>> >     mempty = NoMyData
>> >     mappend NoMyData a = a
>> >     mappend a NoMyData = a
>> >     mappend (MyData a) (MyData b)|a == b = MyData a
>> >                                  |otherwise = NoMyData
>> >
>> > but not for Alternative, which is "(* -> *) -> Constraint".
>> >
>> > *ORIGINAL ERROR DUMP*:
>> > ======================
>> >     42  16 error           error:
>> >                                • No instance for (Eq a) arising
>> > from a use of ‘==’ Possible fix:
>> >                                    add (Eq a) to the context of
>> >                                      the type signature for:
>> >                                        (<|>) :: Origin a -> Origin
>> > a -> Origin a • In the expression: a == b
>> >                                  In the expression: if a == b then
>> > NoOrigin else NoOrigin In an equation for ‘<|>’:
>> >                                      a <|> b = if a == b then
>> > NoOrigin else NoOrigin (intero)
>> >
>> >
>> > /Best regards
>> >   Paul
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