If you want an example, consider writing a simple recursive sudoku solver, where at each location you fill in one empty square with the values 1-9, and reject e

If you want an example, consider writing a simple recursive sudoku solver, where at each location you fill in one empty square with the values 1-9, and reject early if you ever have a repeated value in a row, column or box. This is easy to write in C, and I always imagined would be something functional programming would excel at, but it doesn't seem to fit with your rules.