From: Run Paint Run Run <redmine@...>
Date: 2009-10-14T20:07:52+09:00
Subject: [ruby-core:26087] [Bug #2212] Using a Lambda with Inappropriate Arity for Hash#default_proc=

Bug #2212: Using a Lambda with Inappropriate Arity for Hash#default_proc=
http://redmine.ruby-lang.org/issues/show/2212

Author: Run Paint Run Run
Status: Open, Priority: Low
Category: core
ruby -v: ruby 1.9.2dev (2009-10-14 trunk 25330) [i686-linux] 

If the default_proc of a Hash is a lambda whose arity != 2, key lookups raise an ArgumentError. To understand this exception the user must know that the hash has a default proc, lambdas perform strict argument checking, and that the lambda is passed two arguments.

Instead, perhaps Hash#default_proc and Hash.new could raise a helpful error message if they're passed a lambda with an arity other than 2. This localises the exception to where the default_proc is set, as opposed to where an element is referenced, and better explains the problem.
 
  >> h={}
  => {}
  >> h.default_proc=->{ true }
  => #<Proc:0x9eaaea8@(irb):12 (lambda)>
  >> h[:key]
  ArgumentError: wrong number of arguments (2 for 0)
	from (irb):13:in `yield'
	from (irb):13:in `default'
	from (irb):13
	from /usr/local/bin/irb:12:in `<main>'


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