One of the corners I often paint myself into when developing a tool is only accepting one type of input, usually STDIN, the standard input stream, like a pipeline (ex: cat fruit.txt | grep apple) or a redirect (ex: grep apple < fruit.txt)
What inevitably happens is I end up wanting the tool to work like any Unix tool and accept different kinds of input (filenames or arguments on the command line, for example.)
Finally I got fed up with it and added a function called multi_input()/users/tinypig/2015/08/index.html