Ruby: why do while and until not return last line they execute from a function?

Question

I expect a while loop to return the last statement it executes, but a function does not seem to return that.

(1) This seems to work..

[10] pry(main)> counter = 0
=> 0
[11] pry(main)> a = counter+=1 while counter < 10
=> nil
[12] pry(main)> a
=> 10

(2) This does not work as I expected. I expect 10 to be returned and stored into b.

[19] pry(main)> def increment(terminal_value)
[19] pry(main)*   counter = 0  
[19] pry(main)*   while counter < terminal_value
[19] pry(main)*     counter+=1
[19] pry(main)*   end  
[19] pry(main)* end  
=> :increment
[20] pry(main)> b = increment(10)
=> nil
[21] pry(main)> b
=> nil

Questions:

• Why, in (1), does nil get returned from the assignment statement?
• Why does b not get assigned 10?

Update:

As @DaveNewton mentioned, in (1), I thought I was doing:

a = (counter +=1 while counter < 10)

but I was actually doing:

(a = counter +=1) while counter < 10

Answer 1

In both your examples, the while loop results as nil.

From while loop:

The result of a while loop is nil unless break is used to supply a value.

The same for until:

Like a while loop the result of an until loop is nil unless break is used.