syntax - In Ruby, why is a method invocation not able to be treated as a unit when "do" and "end" is used?

Question

The following question is related to the question "Ruby Print Inject Do Syntax". My question is, can we insist on using do and end and make it work with puts or p?

This works:

a = [1,2,3,4]

b = a.inject do |sum, x|
  sum + x
end
puts b   # prints out 10

so, is it correct to say, inject is an instance method of the Array object, and this instance method takes a block of code, and then returns a number. If so, then it should be no different from calling a function or method and getting back a return value:

b = foo(3)
puts b

or

b = circle.getRadius()
puts b

In the above two cases, we can directly say

puts foo(3)
puts circle.getRadius()

so, there is no way to make it work directly by using the following 2 ways:

a = [1,2,3,4]

puts a.inject do |sum, x|
  sum + x
end

but it gives

ch01q2.rb:7:in `inject': no block given (LocalJumpError)
        from ch01q2.rb:4:in `each'
        from ch01q2.rb:4:in `inject'
        from ch01q2.rb:4

grouping the method call using ( ) doesn't work either:

a = [1,2,3,4]

puts (a.inject do |sum, x| 
        sum + x   
      end)

and this gives:

ch01q3.rb:4: syntax error, unexpected kDO_BLOCK, expecting ')'
puts (a.inject do |sum, x|
                 ^
ch01q3.rb:4: syntax error, unexpected '|', expecting '='
puts (a.inject do |sum, x|
                          ^
ch01q3.rb:6: syntax error, unexpected kEND, expecting $end
      end)
         ^

finally, the following version works:

a = [1,2,3,4]

puts a.inject { |sum, x|
    sum + x
}

but why doesn't the grouping of the method invocation using ( ) work in the earlier example? What if a programmer insist that he uses do and end, can it be made to work?

Answer 1

From the (unofficial) ruby grammar, we see that the contents of (...) in puts (...) must be CALL_ARGS, which don't directly reduce to STMT. However, they can reduce to '(' COMPSTMT ')'. By including an extra set of parentheses, you can use do ... end.

a = [1,2,3,4]

puts ((a.inject do |sum, x| 
         sum + x   
       end))