Symbol#to_proc example

coercing_closures.rb

#http://pragdave.pragprog.com/pragdave/2005/11/symbolto_proc.html
#http://ruby-doc.org/core-2.0.0/Symbol.html#method-i-to_proc

2.0.0p247 :001 > a = ["first", "second", "third"]
 => ["first", "second", "third"] 
2.0.0p247 :002 > a.collect(&:upcase)
 => ["FIRST", "SECOND", "THIRD"] 
2.0.0p247 :003 > a.collect{ |n| n.upcase }
 => ["FIRST", "SECOND", "THIRD"] 
2.0.0p247 :004 > 

a.collect(&:upcase) works but a.collect(:upcase) does not work
however
a.reduce(&:+) and a.reduce(:+) both work
2.0.0p247 :008 > a.collect(:+)
ArgumentError: wrong number of arguments (1 for 0)