For a checkers board [[R|B|E, ...], ...] where

• R represents a red piece
• B represents a black piece
• E represents an empty space

determine if any piece is in danger of attack.

Adapted from a solution I wrote in Q. Wanted to see how the array-centric concepts from APL/J/K/Q worked in Haskell. Additionally benefits from ADTs and Monads of course. Attempted to be as tacit as possible. Presented as a fun curiosity.