Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Functional programming, design and analysis of algorithms / 3 / c

First enumerate each possible row, then choose one row from each row's alternatives:

expand :: Matrix [a] -> [Matrix a]
expand = cp . map cp

The inner Cartesian product converts one row of cell choices into its possible rows; the outer product combines those rows into whole matrices. Every assignment is represented exactly once when each cell's alternatives are distinct. Empty cell domains yield no grids, and an empty matrix yields the singleton list containing the empty matrix.