[labrador]

Whenever someone mentions state I think of Rich Hickey's talk "Simple Made Easy" in which he seems almost terrified of state and strives always for pure functions, because as a human he has a hard limit as to how much state he can keep in his head at once when reasoning about the program. He's right. I've worked on enough bad code, including sometimes my own, to know that programmers are bad at managing state.

"Simple Made Easy" - Rich Hickey (2011) https://www.youtube.com/watch?v=SxdOUGdseq4

[karmakaze]

Pure functions are useless without inputs and outputs. The missing piece of the puzzle here is that we only want transitions between valid states. One way to define valid states is by enforcing invariants which must always be true. Armed with these, all the stuff that happens in-between can be guarded against, much like constraints in a database schema.

Contrast this with non-pure functions which incrementally mutate from a valid state, to a sequence of invalid states, and end up at a valid state again. If something goes wrong along the way, we end up with an invalid state. Think about program state like it's a journaling file system.

[warkdarrior]

That just developers being lazy! As a user, I want my computer and my software to manage all state for me. This means remembering all data I entered (so I can continue work from where I left off), all files I saved (so I can search for them easily), all actions I've taken (so I can undo them), all sites I browsed (so I can find them again), all info I uploaded (so I can check which website knows what about me), etc.

[karmakaze]

I also found this to be defeatist (and the basis of the post):

> As a programmer, it’s impossible to predict all the states that your program can end up in.

It's true that we can't predict all the things other programmers will change the program to do in the future, but we can take precautions such as rejecting invalid states, or if being liberal in the input it accepts transform it to something usable.