How to read the contents of a file In Erlang?

You could leverage file:read_file/1 and binary:split/3 to do this work in two steps:

readlines(FileName) ->
    {ok, Data} = file:read_file(FileName),
    binary:split(Data, [<<"\n">>], [global]).

file:read_file/1 is what you are looking for. Just for teaching purpose, Accum ++ [Line] is bad practice. Problem is that left argument of ++ is copied and right is used as is. In your code you will copy bigger and bigger part in each iteration. Solution is lists:reverse(Line,Accum) and than return lists:reverse(Accum) in your eof branch (Or [Line|Accum] and lists:append(lists:reverse(Accum)) at the eof or use binary which have better append operation or ...). Another way is not using tail recursive function which is not so bad as seems at first time according to Myth: Tail-recursive functions are MUCH faster than recursive functions.

So your readlines/1 function should look like

readlines(FileName) ->
    {ok, Device} = file:open(FileName, [read]),
    try get_all_lines(Device)
      after file:close(Device)
    end.

get_all_lines(Device) ->
    case io:get_line(Device, "") of
        eof  -> [];
        Line -> Line ++ get_all_lines(Device)
    end.