Curry a variadic function

Fixed-arity currying can watch fn.length to know when it has enough. A variadic function has fn.length of 0, so there is nothing to count towards, and the curried form needs an explicit end signal instead. The convention is a call with no arguments, which is also the trick behind the add(1)(2)(3) puzzle.

Implement variadicCurry(fn). It returns a collector function. Calling the collector with one or more arguments returns a new collector holding everything gathered so far, so arguments can be supplied one at a time or several at a time, over any number of calls. Calling a collector with no arguments invokes fn with all of the arguments gathered along that chain and returns its result — so variadicCurry(fn)() invokes fn with no arguments at all. Every collector is independent: branching off the same partially applied collector twice must give two separate results, with no arguments leaking between the branches.

What it has to do

• Collect arguments across any number of calls, one or several at a time.
• Invoke fn with everything collected when a collector is called with no arguments.
• Invoke fn with no arguments when the first call is empty.
• Keep branches independent: reusing a partially applied collector must not accumulate arguments across uses.

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