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Feature #10617

Change multiple assignment in conditional from parse error to warning

Added by recursive-madman (Recursive Madman) almost 6 years ago. Updated about 3 years ago.

Status:
Closed
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:66944]

Description

There is currently an inconsistency between regular and multiple assignment in conditionals.
Regular assignment causes a warning, multiple assignment causes a parse error.

The historical reason for this is that in 1.8 multiple assignment would always return an Array, but since 1.9 it returns whatever the RHS evaluates to.

Examples:

a, b = nil  #=> nil
a, b = []   #=> []     (but a and b are both nil)
a, b = 1,2  #=> [1, 2]

Since multiple assignment behavior has changed, it makes sense to remove the (artificial) parse error for multiple assignments.

That makes it possible to test the return value of a method used for multiple assignment without having to use a temporary variable.

Example:

# CURRENTLY WORKING CODE:
tmp = some_method_returning_array_or_nil
a, b = tmp
if tmp
  # method returned an array (possibly empty)
else
  # method returned nil.
end

# PROPOSED WORKING CODE:
if(a, b = some_method_returning_array_or_nil)
  # method returned an array (possibly empty)
else
  # method returned nil
end

(the parenthesis are needed due to LALR limitations, as discussed in #10450)

Attached is a patch that does the necessary change.


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