I don't personally use functional programming languages on a daily basis, but this, I feel, is the veil between programmers who are highly successful and those

I don't personally use functional programming languages on a daily basis, but this, I feel, is the veil between programmers who are highly successful and those who limp along: being able to have a mathematical (side effect free) approach to solving problems. This requires being able to break a larger problem into subsets. The closer you can get your methods to not having affects on the global scope, the better a programmer you will become. In other words, you don't have to be able to keep the whole problem in you head, but you have to be able to recognize how to subdivide the problem into manageable bits. The other major trap that I think affects programmers is not appreciating what is happening when you transform your data: when you make a change to data state, how does that affect the system? Where are you persisting that change? Do you need to persist a change or is it simply a temporal snapshot of something happening? Basically if somebody claims to be a "good" programmer, but they don't have a solid grasp on set theory, they I'm sceptical that they will be a "good" programmer in the long run. Naturally you also have to become familiar with the language you are using and the environment you are developing for.