Bug #9776: Ruby double-splat operator unexpectedly modifies hash

• Status: Closed
• Assignee: matz (Yukihiro Matsumoto)
• Target version: 2.2.0
• ruby -v: ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-linux]
• Backport: 2.0.0: DONTNEED, 2.1: DONE

[jessesielaff (Jesse Sielaff)]

I noticed what I find to be a very surprising behavior with the double-splat (**) operator in Ruby 2.1.1.

When key-value pairs are used before a **hash, the hash remains unmodified. However, when key-value pairs are only used after the **hash, the hash is permanently modified.

h = { b: 2 }

{ a: 1, **h }        # => { a: 1, b: 2 }
h                    # => { b: 2 }

{ a: 1, **h, c: 3 }  # => { a: 1, b: 2, c: 3 }
h                    # => { b: 2 }

{ **h, c: 3 }        # => { b: 2, c: 3 }
h                    # => { b: 2, c: 3 }

For comparison, consider the behavior of the splat (*) operator on arrays:

a = [2]

[1, *a]     # => [1, 2]
a           # => [2]

[1, *a, 3]  # => [1, 2, 3]
a           # => [2]

[*a, 3]     # => [2, 3]
a           # => [2]

The array remains unchanged throughout.

---

Tsuyoshi Sawada has also highlighted that the expression's result is the self-same object as the original hash:

h.object_id == { **h, c: 3 }.object_id # => true

---

I investigated parse.y to try to determine the error there, but I couldn't narrow it down any further than the list_concat or rb_ary_push function calls in the assocs : block of the grammar.

Without exhaustively examining the C source, I think the best clue to the mechanism behind the erroneous behavior might be the following:

h = { a: 1 }
{ **h, a: 99, **h } # => {:a=>99}

That we don't see {:a=>1} illustrates that h[:a] is already overwritten by the time the second **h is evaluated.

---

Here is the use case that led me to this discovery:

def foo (arg) arg end

h = { a: 1 }

foo(**h, b: 2)

h # => { a: 1, b: 2 }

In the above example, I don't want { b: 2 } permanently added to my existing hash. I'm currently solving it like this:

h = { a: 1 }

foo(**h.dup, b: 2)

h # => { a: 1 }

The call to #dup feels unnecessary, and is inconsistent with the analogous behavior when using the single * operator. If this bug is fixed, I'll be able to eliminate that call.

[jessesielaff (Jesse Sielaff)]

Origin:

http://stackoverflow.com/questions/23282342/ruby-double-splat-operator-destructive-vs-nondestructive

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN to 2.0.0: DONTNEED, 2.1: REQUIRED

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

Applied in changeset r45724.

---

compile.c: non-destructive keyword splat

• compile.c (compile_array_): make copy a first hash not to modify
  the argument itself. keyword splat should be non-destructive.
  [ruby-core:62161] [Bug #9776]

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.0.0: DONTNEED, 2.1: REQUIRED to 2.0.0: DONTNEED, 2.1: DONE

Backported into ruby_2_1 branch at r46451.