Can't figure out why these multidimensional access behave so differently

[fsicre]

https://play.vlang.io/?query=b2511098b6

Could you find the right documentation for me ?

[jorgeluismireles]

Add mut to this:

pub fn (grid Grid) at(i int, j int) &Cell {
    return &(grid.cells[i][j])
}

so at() call can modify Grid:

pub fn (mut grid Grid) at(i int, j int) &Cell {
    return &(grid.cells[i][j])
}

[JalonSolov]

at() doesn't modify grid, it just returns a cell.

[jorgeluismireles]

@JalonSolov the mut above worked for me, seems grid.cells returned is a reference whose fields can be updated outside the function at()