Ruby 3.0 Interpolated Strings Are No Longer Frozen

September 16, 2020

In Ruby, frozen_string_literal: true makes all string literals frozen by default and it helps in reducing needless memory allocations by not creating a new allocation each time a string is redefined.

In Ruby 2.7.1 and earlier versions, interpolated strings were also considered to be frozen when frozen_string_literal: true is used.

# frozen_string_literal: true

foo = "Foo"       # frozen
bar = "Bar"       # frozen
"#{foo} #{bar}"   # frozen

This was intentionally done for simplicity that all strings with "" or '' are frozen by default when frozen_string_literal: true is present.

But an interpolated string is not actually a string literal but rather it can be called as a dynamic string.

Let's see this example:

# Ruby 2.7.1
# frozen_string_literal: true

def full_name(first_name, last_name)
    "Mr. #{first_name} #{last_name}"
end

fname = "Dummy"
lname = "Rik"
name = full_name(fname, lname)

puts fname.frozen?
#=> true

puts lname.frozen?
#=> true

puts name.frozen?
#=> true

puts name.gsub!(/Mr/, "Mrs")
#=> `gsub!': can't modify frozen String: "Mr. Dummy Rik" (FrozenError)

As we can see above, name is an interpolated string i.e dynamic string.
full_name method by default returns a frozen string as Ruby makes interpolated strings frozen by default when frozen_string_literal: true is used.

Let's fix the above FrozenError :

# Ruby 2.7.1
# frozen_string_literal: true

def full_name(first_name, last_name)
    interpolated_string = "Mr. #{first_name} #{last_name}"

    puts "interpolated_string frozen? : #{interpolated_string.frozen?}"
    puts "interpolated_string object_id : #{interpolated_string.object_id}"

    dup_string = interpolated_string.dup
    puts "dup_string frozen? : #{dup_string.frozen?}"
    puts "dup_string object_id : #{dup_string.object_id}"

    dup_string
end

fname = "Dummy"
lname = "Rik"
name = full_name(fname, lname)

puts name.gsub!(/Mr/, "Mrs")

#=> interpolated_string frozen? : true
#=> interpolated_string object_id : 60
#=> dup_string frozen? : false
#=> dup_string object_id : 80
#=> Mrs. Dummy Rik

We have used dup to return a non freezing string, so that we can use gsub! on it.

But we can see above (interpolated_string object_id : 60, dup_string object_id : 80), we have created two memory allocations unnecessarily.

As said above, we use frozen_string_literal: true to reduce memory allocations but here we are creating extra memory allocation.

Ruby core team has accepted a proposal to not to freeze interpolated strings when frozen_string_literal: true is used and this has been merged to Ruby 3.0 .

Let's for the final time see how our example will shape with Ruby 3.0 :

# Ruby 3.0
# frozen_string_literal: true

def full_name(first_name, last_name)
  "Mr. #{first_name} #{last_name}"
end

fname = "Dummy"
lname = "Rik"
name = full_name(fname, lname)

puts name.gsub!(/Mr/, "Mrs")
#=> Mrs. Dummy Rik

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