fromList doesn't guarantee a rectangular matrix

[maurisvh]

As-is, the fromList implementation does not check if the given list of lists is a rectangle; this can break lots of things.

Often, of course, you know for sure that the lists you're passing in are a rectangle, but there should probably be a warning about the fact that elm-matrix expects this in the docs.

Perhaps there should also be a List (List a) -> Matrix (Maybe a) function, that pads rows shorter than the longest one with Nothings.

[sindikat]

I agree that this is bad, it's user's fault, if they create a Matrix manually by relying on implementation details, but the API should be safe, and fromList is part of API. fromList shouldn't ever return junk, whatever the data.

I think the best way is to throw away elements exceeding width of the matrix, where width is a length of the shortest sublist. Something like:

fromList : List (List a) -> Matrix a
fromList xs =
  let
    minWidth = List.minimum <| List.map List.length xs
  in
    Array.map (Array.fromList << List.take minWidth) <| Array.fromList xs

I also agree that function of type List (List a) -> Matrix (Maybe a) is a good idea. I want this library to be very inclusive.

Another idea I had is fromList of type List (List a) -> Maybe (Matrix a), which returns Nothing if gets non-rectangular list of lists. But I am against this idea for now, because from my experience with Elm so far Maybe is a pain. Yes, there is andThen and withDefault etc, but unless I can figure out how to use pervasive Maybe without constantly writing spaghetti case expressions, I don't want to rely on Maybe so much.

The biggest weakness of the library is that Matrix type is not opaque, it's just a type synonym. So anyone can create an erroneous array of arrays and pretend this is a Matrix. Implementation details should be hidden. Moreover, I find implementation of a Matrix using 1-dimensional array (with explicit tracking of width and height) more elegant and possibly more efficient. I want to defer reimplementation of Matrix until the library accumulates more functions and is more battle-tested.