squarism squarism - 10 days ago 7
Ruby Question

Why does double shovel in Ruby not mutate state?

I ran into this weird side effect that caused a bug or confusion. So imagine that this isn't a trivial example but an example of a gotcha perhaps.

name = "Zorg"

def say_hello(name)
greeting = "Hi there, " << name << "?"
puts greeting
end

say_hello(name)
puts name

# Hi there, Zorg?
# Zorg


This doesn't mutate name. Name is still
Zorg
.

But now look at a very subtle difference. in this next example:

name = "Zorg"

def say_hello(name)
greeting = name << "?"
puts "Hi there, #{greeting}"
end

say_hello(name)
puts name

# Hi there, Zorg?
# Zorg? <-- name has mutated


Now name is
Zorg?
. Crazy. Very subtle difference in the
greeting =
assignment. Ruby is doing something internally with the parsing (?) or message passing chaining? I thought this would just chain the shovels like
name.<<("?")
along but I guess this isn't happening.

This is why I avoid the shovel operator when trying to do concatenation. I generally try to avoid mutating state when I can but Ruby (currently) isn't optimized for this (yet). Maybe Ruby 3 will change things. Sorry for scope-creep / side discussion about the future of Ruby.

I think this is particularly weird since the example with less side effects (first one) has two shovel operators where the example with more side effects has fewer shovel operators.

Update
You are correct DigitalRoss, I'm making it too complicated.
Reduced example:

one = "1"
two = "2"
three = "3"
message = one << two << three


Now what do you think everything is set to? (no peeking!)
If I had to guess I'd say:

one is 123
two is 23
three is 3
message is 123


But I'd be wrong about two. Two is 2.

Answer

If we convert your a << b << c construct to a more method-ish form and throw in a bunch of implicit parentheses the behavior should be clearer. Rewriting:

greeting = "Hi there, " << name << "?"

yields:

greeting = ("Hi there, ".<<(name)).<<("?")

<< is modifying things but name never appears as the target/LHS of <<, the "Hi there ," << name string does but name doesn't. If you replace the first string literal with a variable:

hi_there = 'Hi there, '
greeting = hi_there << name << '?'
puts hi_there

you'll see that << changed hi_there; in your "Hi there, " case, this change was hidden because you were modifying something (a string literal) that you couldn't look at afterwards.