[adrianm]

This is so true - what interesting to me is that I have heard this sentiment directly from friends and coworkers before several times. Upon further prodding, it has been the case that their dealings Haskell were minor at best, and are confusing the superficially similar syntax of Haskell with languages they're used to, thereby giving them the idea they understand the code they're reading. But more often than not they really don't understand it - the brain is apparently adept at making us think we understand something we really don't based on a superficial, usually structural similarity with an existing knowledge domain we have.

I recall interviewing a potential hire who lauded his appreciation and understanding of functional programming - and he really believed it. So I asked him to explain to me what a closure was, and give me some examples of ways you can exploit them in your code, practically. Pretty straightforward question for someone who claims to understand the concepts of functional programming.

Of course, I wouldn't be bringing this up if he was even remotely successful, but I don't think this was a result of his presenting himself in a dishonest manner. I think closures are subtle ideas; much like function application, composition, and other ideas that seem familiar and easy to understand until you are asked to apply them practically. That's when the gap between what you think you know and what you actually know is borne for the world to see.

[auggierose]

Closures are nice, but you don't have to understand them in order to understand functional programming ... Closures are more about how functional programming languages are implemented.

[adrianm]

I find encapsulating mutable state within closures to be an indispensable and practical tool I use in almost every (stateful) program I write.