Mentioning just constrained fields inside instance of typeclass

Question

Hello i want to know is there any way having a type that implements a typeclass to put a constraint on a field without decomposing all the type?(or simply put without enumerating all its other fields in the instance declaration? )

Example:

data A=A | BB | CCC deriving( Show)

class T a where
  mymethod::a->Bool

instance T A where
 mymethod a = length . show $ a >1

data Complex b = Complex{
                  a::Int,
                  b::A,
                  c::String
                  }

instance (T b) => T (Complex a b c)

Looking above in the last line, is there any way to enumerate just the fields that we want to put constraints on? (in our case b which implements T typeclass).
Can we put wildcards or any other thing to not put all fields?

E.g:
instance (T b) => T (Complex _ b _)
or even better

 instance (T b) => T Complex {b ? }

but here there is only one type parameter for complex: an a. The number of parameters of the data constructor is three, but of the type constructor (instances work on that meta-level), there is one parameter. — Willem Van Onsem
Furthermore this a has no use here, since you never specify a field with a type that is defined in terms of a. — Willem Van Onsem
I am not sure i am following.I made some edits in the meantime. — Bercovici Adrian
The field inside Complex that implements the T typeclass is b. — Bercovici Adrian
No, the b parameter has type A which is a real type, not a... — Willem Van Onsem

Daniel Wagner Daniel Wagner · Accepted Answer · 2018-07-12T13:49:31

After fixing your implementation of mymethod for A (you need parentheses around the first argument to >), the following works for me:

data Complex = Complex{
                  a::Int,
                  b::A,
                  c::String
                  }

instance T Complex where
    mymethod c = mymethod (b c)

I have removed the type parameter to Complex -- I suspect you were getting confused and believing there was a connection between the type named b and the field named b when there wasn't. If you want, you could make that connection explicit (here I'll use different names to emphasize how the connection is made, but you could drop the Tys and Fields below and still have valid code):

data Complex aTy bTy cTy = Complex
    { aField :: aTy
    , bField :: bTy
    , cField :: cTy
    }

instance T bTy => T (Complex aTy bTy cTy) where
    mymethod val = mymethod (bField val)

Mentioning just constrained fields inside instance of typeclass

1 Answers