-- Neal Stephenson, Zodiac (1988) http://en.wikiquote.org/wiki/Neal_Stephenson#Zodiac_.281988....
(I'd include the previous paragraphs in the quote, to show the conversation the protagonist actually has about this, if my copy wasn't on loan to a friend right now, dammit! If anyone else can post that, that's be cool.)
Then, when your manager inevitably does not believe you, ask him if he thinks you missed something (no), then ask him if he trusts you (yes), then contemplate the face of cognitive dissonance.
Norvig it covers this well in his Teach Yourself Programming in Ten Years (http://norvig.com/21-days.html) essay.
Coming from a strong background of similar languages? Sorry, you're wrong there. Perhaps you aren't familiar with Erlang; it is the "everyman" of functional languages. The things I think you'd call "idioms" (not really sure what you mean as that's a very loose term), let's say recursive programming, pattern matching, carry over as-is from any number of other functional languages. Pattern matching? It's a mix of OCaml and Prolog. Terms? Imagine Scheme had tuples in addition to lists. Etc. The only "new" concept is the message system, which if you know anything about networking, is pretty straightforward.
In other words, if you know pretty much any other functional language, it's almost trivial to translate basic programs to & from into Erlang knowing little more than its syntax, which as you concede, is simple.
OTOH, if you're coming from, say, PHP, sure, learning Erlang will be difficult as first you have to understand functional programming. But you're asserting impossibility, so counter-examples are moot.
Ten Years… I've been coding over twenty years in more languages than I can count… I have a good idea how long it takes to learn a language well. Maybe I'm biased because it's the latest language I've studied, but Erlang was the first non-toy language where I skimmed the reference manual, said "huh, that was all unsurprising", and started coding effectively.
One problem I have, which Norvig doesn't talk about, is that I'm currently moving from one ecosystem (Microsoft) to another (the Java world), and it's not just about the language, it's everything, all the toolchain is different and unfamiliar.
The way of doing things is quite different too.
Learning Cocoa or Cocoa Touch, otoh...
I'll have to check erlang out. I only hear good things.
A) Writing a "Hello, World"?
B) being able to fix a simple bug in a program?
C) Writing a small program that solves some domain problem
D) Writing a program that solves a "real world" problem, making use of the programming languages' strengths and conforming to standards (something that other people would let you commit into a repository unchallenged :) )
I'm certain A) takes only a few hours, and B) can be done in a day.
As Norvig says, you can do C) if you write in the new language like you would in a language you already know (not much of a problem if we're talking C# vs Java).
But D) IMO takes weeks/months.
Ignoring OTP, my ability to use the language proper was pretty much complete within a week, owing to its simplicity and my prior background in functional languages.
This is in contrast to C, which I have been using for around 15 years now, yet am still learning nuances of the language. (Things like: which integer operations are undefined on negative numbers; how integer promotion works with shift operators; how "restrict" interacts with scope.)
C is almost a fractal of nuance that does take years of experience to comprehend; Erlang has no nuance. (The closest thing to nuance I can think of is the relationship between integers and floats; and even then the takeaway is "it just works; don't worry about it". The only language in which I've seen the number hierarchy handled more cleanly is Racket.)
"Experiments that show that Erlang is N times better than "something else" won't be believed if N is too high."
That is exactly it. I've had those arguments where I was forced to tell a manager about another project I did for another company, a similar project where things went quickly and smoothly, using technology the manager was not familiar with. And every word I spoke was met with disbelief.