Underscore doesn't work in Coffeescript's console

Question

I've just started using Coffeescript and the Coffeescript console, and Underscore. However, whenever I define a function, Coffeescript decides that _ means that function, and seems to forget the initial assignment of _ = require 'underscore'.

Why does this happen? How can I prevent it?
(I'd really like to be able to copy paste code from my files into the console.)

(Does _ have some special meaning in the Coffeescript console? Does it mean "the last result" or something? That'd explain my issue?)

Details:

$ coffee 
coffee> _.contains [1, 2, 3], 3   # no Underscore, initially
TypeError: Cannot call method 'contains' of undefined
    at ...
coffee> 
coffee> _ = require 'underscore'
{ [Function]
  _: [Circular],
  VERSION: '1.3.3',
  forEach: [Function],
  ...

coffee> _.contains [1, 2, 3], 3    # now Underscore works fine
true
coffee> 
------> someFunction = (a, b) ->   # define a function ...
......>   a + b

[Function]
coffee> 
coffee> _.contains [1, 2, 3], 3     # now `_` is not Underscore any more!
TypeError: Object function (a, b) {    # Does `_` mean "last result" or sth?
  return a + b;
} has no method 'contains'
    at evalmachine.<anonymous>:3:7
    at Object.eval (/usr/local/lib/node_modules/coffee-script/lib/coffee-script/coffee-script.js:142:17)
    at Interface.<anonymous> (/usr/local/lib/node_modules/coffee-script/lib/coffee-script/repl.js:131:40)
    at Interface.emit (events.js:67:17)
    at Interface._onLine (readline.js:162:10)
    at Interface._line (readline.js:426:8)
    at Interface._ttyWrite (readline.js:603:14)
    at ReadStream.<anonymous> (readline.js:82:12)
    at ReadStream.emit (events.js:88:20)
    at ReadStream._emitKey (tty.js:327:10)
coffee> 
coffee> _ = require 'underscore'
coffee> _.contains [1, 2, 3], 3    # Now all is fine again, for a short while
true

Answer 1

The CoffeeScript REPL has this JavaScript at its heart:

try {
  _ = global._;
  returnValue = CoffeeScript["eval"]("_=(" + code + "\n)", {
    filename: 'repl',
    modulename: 'repl'
  });
  if (returnValue === void 0) {
    global._ = _;
  }
  repl.output.write("" + (inspect(returnValue, false, 2, enableColours)) + "\n");
} catch (err) {
  error(err);
}

So if the last command returned something then _ will be that something. I can't find any documentation about this though but searching for _ isn't a terribly productive activity. If you want to use Underscore.js in the CoffeeScript REPL, you'll have to call it something other than _.

Thanks to Trevor Burnham (who wrote the book so I think we can trust him on this) we know that the CoffeeScript REPL uses _ as the last result to match the behavior of the node.js REPL:

REPL Features
[...]
The special variable _ (underscore) contains the result of the last expression.

Ruby's irb does the same thing.