Structure your shell script with monad stack

We write a lot of shell scripts during our career as a programmer. If you read them with a little carefulness, you will find that almost every command within the shell script is conducting some effect to read something from outside or make some change to outside world. These scattered effect would make reasoning about your script very difficult especially when debug a long script.

You may wonder if there is a better way to structure a script. The answer is yes, there is a better way to do this and it is the FP way.