Few days ago I was working on some API handlers in our server written in Haskell with the framework Yesod. There was a mistake, some secret data was accessible to normal users because we forgot to check the user's type, luckily it's not on production yet.
This is an article explaining the concept of functor to programmers who are interested in Haskell but not sure how to get started on those abstract ideas.
Many articles introduce functional programming with examples of maping, filtering and reduceing arrays, but it feels like it is not very useful except from array manipulation.
This post focus on the map concept and try to explain how is it useful in other ways.
(This is the 10th post for Haskell Advent Calendar 2017)