Base1.hs

-- https://github.com/Dobiasd/articles/blob/master/from_oop_to_fp_-_inheritance_and_the_expression_problem.md

module Base where

import Data.List

data Base = Foo Int | Bar String

step (Foo intVal) delta = Foo $ intVal + delta
step (Bar strVal) delta = Bar $ strVal ++ show delta

display (Foo intVal) = show intVal
display (Bar strVal) = strVal

stepAll l = map (\b -> step b 1) l

displayAll l = putStrLn $ concat (intersperse "\n" $ map display l)

main =
 displayAll ((stepAll . stepAll . stepAll) [Foo 0, Bar ""])