In shell, you can do it in many ways:

cat input | xargs echo | tr ' ' ,

or

perl -pe 's/\r?\n/,/' input > output

I know you wanted this to be longer, but I don't really see the point of writing multi line script to solve such simple task - simple regexp (in case of perl solution) is fully workable, and it's not something artificially shortened - it's the type of code that I would use on daily basis to solve the issue at hand.