[tinco]

People are talking about monads and stuff like that. No need to worry about maths and words you don't need to know. That 'do' keyword up there indicates the start of a simple DSL. The DSL goes like this, every line is the beginning of a lambda. And the result of each lambda evaluation is passed into the next lambda.

So you get a sort of cascading scope of lambdas where the result of each lambda is available passed into the next. Each lambda depends on the evaluation of the previous one. Normally in Haskell functions are executed lazily, this structure forces the sequential evaluation.

So what are these cd, mkdir, output etc functions? They return an object with a specific type called 'IO'. This type is monadic, but that's irrelevant for now. Haskell as you know has no side effects in the language itself. The IO type basically is a command pattern, it says "execute this I/O with these parameters".

The monadic aspect of IO makes it so that at the end the commands will have accumulated in a list, of which you can get an item if you give it the results of the previous item. So that's what the main function returns, a list of commands with some lazily evaluated Haskell code in between them. Now comes the side effect part. The Haskell runtime system iterates over the list of commands and executes them. The result of each command is used to get the next command in the list.

So that's the core of the magic trick of monadic I/O, you make a lazy list of I/O commands, and have something external to the language execute those I/O commands, giving the results back to the language to get the next I/O command to execute.