can I make a list of types in haskell?

Question

I have different data types that I have defined, and I want to make them all an instance of a class. is it possible to make a list of types and map instance over them instead of having to declare them all individually?

I mean something like this:

data Type1 = ...
data Type2 = ...

map (instance ClassName) [Type1, Type2]

Answer 1

Well actually you can, with something like

{-# LANGUAGE TemplateHaskell #-}

module T where

class C a

data X = X
data Y = Y
data Z = Z

$(fmap concat $ mapM (\t -> [d|instance C $t|]) [[t|X|], [t|Y|], [t|Z|]])

but it strikes me as massive overkill unless you actually need them to be generated automatically (e.g., the list of types might vary depending on something).