[Haskell-beginners] Replace instances of module

[Baa]

Thanks very much, David and Francesco! I got your solutions.

> The reason you don't get to override instances within a package is
> that haskell accounts for the fact that a library user could import
> both old and new, ending up with two conflicting instances for the
> same type.
> 
> If one of the types now needs two different JSON instances, one way
> to do that is to wrap it in a newtype that has a different JSON
> instance.
> 
> data MyType = ... -- old type
> 
> newtype MyType_v2 = MyType_v2 MyType
> -- data MyType_v2 = ... -- alternatively, define same structure, if it
> didn't change.
> 
> instance JSON MyType_v2 = ... - slightly different json instance.
> 
> Then you can have a module that has a function that returns MyType_v2
> from its functions instead and exports a JSON instance for it.  Users
> can import either or both and will have access to both instances and
> both types.
> 
> You could also do it in the reverse, with a _v1 type, but old clients
> will have to be updated.
> 
> As an addendum, you might future proof your API a little by having a
> phantom type variable.
> 
> data V1
> data V2
> data MyType tag = ... -- old type
> 
> v1_func :: MyType V1
> v1_func = undefined
> v2_func :: MyType V2
> v2_func = undefined
> 
> At the cost of a slightly more complicated API for your users.  But
> that won't protect you if MyType itself changes.
> 
> On Mon, Jan 8, 2018 at 9:31 AM, Baa <aquagnu at gmail.com> wrote:
> 
> > Hello, Francesco.
> >
> > Actually the whole problem is that I have library (consisting of
> > several modules: types, API calls, JSON instances, etc) to work with
> > some REST service. Now I have 2 nodes with this service: version 1
> > and version 2, so API is little bit changed in version 2 (mostly in
> > JSON instances, but types seems to be the same, API calls will be
> > the same too).
> >
> > So, clients must work with both services: of version 1 and version
> > 2. And I can create new library, but this will be big copy-paste,
> > because mostly code (except JSON instances) will be the same.
> > Different will be returned value in some API calls, maybe - I'm not
> > sure. Nothing else.
> >
> > So, if I'll create new version of library: how they will live
> > together with the same names, etc, only different version?! And
> > common code-base will lead to complex bug-fixing and test
> > modification. If I'll name new library like "oldlibv2" - problem
> > with bug fixing and testing is still here.
> >
> > My idea was to "patch" in some way existing library by import some
> > modules into new modules and reimplement some instances only. May be
> > the same with API calls, I'm not sure here.
> >
> > I know that it's easy in F#, ML and Python, for example. Are
> > backpacks something like ML modules? I see that they are not
> > supported by stack. What does it mean? "import" will not work?
> >
> > Actually I don;t know - is some better way to solve such problem?
> > I'm sure many of us communicate with some RESTfull APIs. Then APIs
> > change. And your apps should work with old and new APIs (services),
> > so you should clone existing library to support both versions
> > (together, at the same time, in one client application!).
> >
> > ===
> > Best regards, Paul
> >  
> > > Hello Paul,
> > >
> > > On Mon, Jan 08, 2018 at 03:20:20PM +0200, Pv wrote:  
> > > > Hello, all.
> > > >
> > > > Is it possible to import all types except some of instances and
> > > > to re-defined hiding instances, so resulting (new) module will
> > > > be the same types, mostly the same instances, but some of
> > > > instances will be replaced?  
> > >
> > > Nope, instances are automatically imported! You can create
> > > newtypes and have them instances of what you need or check the
> > > newfangled `backpack` [1]!
> > >
> > > [1] https://ghc.haskell.org/trac/ghc/wiki/Backpack
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