Lists of data types: “could not deduce (a ~ SomeType) from the context (SomeTypeclass a)”

Question

I have the following problem with Haskell\'s type system: I am trying to declare a data type and return a list containing elements of this type from a function. Unfortunatel

Answer 1

Haskell does support heterogenous lists of elements, provided their elements are correctly wrapped (see live code on repl.it):

{-# LANGUAGE GADTs #-}

-- your sample type
data SampleType = SampleConstructor

instance Show SampleType where
  show _ = "(SampleType)"

-- MkShowable wrapper
data Showable where
  MkShowable :: Show a => a -> Showable

instance Show Showable where
  show (MkShowable a) = show a


main = do
  let myList :: [Showable]
      myList = [MkShowable SampleConstructor, MkShowable 1, MkShowable True]
  putStrLn $ show myList
  -- ["(SampleType)","1","True"]

See Heterogenous collections for more options.