[weland]

Indeed; I'm used to writing systems software, so user interaction tends to elude me :).

That being said, I would dare say that the business layer is the one which would benefit the most from a FP perspective. I can think of dozens of bugs in my code that originated in my inability to correctly keep track of what was otherwise needlessly exposed state.

Being many layers closer to the silicon, I don't actually use any functional programming language for my work (it's C and Assembly all the way...), so I can't speak for using one. But applying some functional framework to my code proved immensely useful once I started doing it.

(At the risk of sounding like a hipster, that was actually before FP suddenly becoming cool. I tried learning Haskell once and miserably failed; I was only smart enough to learn some Common Lisp).

[Roboprog]

Indeed, many languages would benefit simply from having modifiers that reverse the typical use of "const". Make everything immutable / side-effect-free by default, and add a "mother may I" keyword that allows mutation / side-effects without giving a compile error.

Hopefully, programmers would learn to avoid writing code that requires the "stomps-all-over-shit" keyword except when they really did need it. (assuming tail call elimination for simple loops)

[afiler]

One of D's improvements upon C/C++ is its ability to enforce "deep const-ness".