Re: “If typeclass a, then a is also an instance of b by this definition.”

[Mike Izbicki]

I've also wanted this feature in the past.  I think the
DefaultSignatures language extension is the closest thing.  There's a
recent answer to your stackoverflow question that explains how it
works:

https://stackoverflow.com/questions/3213490/how-do-i-write-if-typeclass-a-then-a-is-also-an-instance-of-b-by-this-definit/44456791#44456791

On Fri, Jun 9, 2017 at 3:15 AM, Saurabh Nanda <saurabhnanda at gmail.com> wrote:
> Hi,
>
> At the outset I would like to mention that I do not completely understand
> how type-class instance resolution works, and why it works that way. I have
> tried reading up, but I don't completely understand WHY it works that way.
> So, please feel free to ask me to RTFM if what I'm about to say doesn't make
> any sense, although TFM doesn't make sense to me!
>
> From a pragmatic standpoint is there any way to express the following idea
> in Haskell (or a future version of Haskell), **with minimal amount of
> boilerplate**
>
>     If a type has an instance of type-class `a`, then here's how you can get
> an instance of type-class `b`
>
> Without the ability to express this idea, you end up with a **lot** of
> type-class boilerplate, (even if you use GHC Generics) eg:
>
>     data State = data State = Inactive | Incomplete | Deleted | Trial |
> Unpaid | Paid
>       deriving (Eq, Show, Generic)
>
>    instance AppEnum State where
>        toString = gEnumToString
>        fromString = gEnumFromString
>
>    -- You are still forced to do the following for **every single type**
> even though, conceptually, this can be driven by a rule
>
>     instance ToJSON State where
>       toJSON s = toJSON $ toString x
>
>     instance FromJSON State where
>       parseJSON (Aeson.String s) = fromString s
>       parseJSON _ = fail "unexpected type"
>
>     instance FromFIeld State where
>       fromField _ mBS = case mBS of
>         Nothing -> fail "not expecting a NULL/Nothing"
>         (Just bs) -> pure $ fromString bs
>
>     instance ToField State where
>       toField s = toField $ toString s
>
> I tried talking to people on IRC and also trawling StackOverflow [1] and my
> take-away is essentially, that Haskell cannot express this idea. However, my
> submission is, that it is a very useful idea to express and probably the
> next version of the language should do something to make the lives of
> programmers a little easier.
>
> [1]
> https://stackoverflow.com/questions/3213490/how-do-i-write-if-typeclass-a-then-a-is-also-an-instance-of-b-by-this-definit
>
> -- Saurabh.
>
>
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