Backport #3938

Ruby incorrectly compares the length of Array elements of Enumerable objects to the arity of Methods given as blocks to Enumerable methods

Added by Ethan (Ethan -) almost 10 years ago. Updated about 1 year ago.



Given an Enumerable object that has an element which is an Array, when an Enumerable method which takes a block (#map, #detect, etc) is called on that object and given a Method as its block, Ruby will incorrectly compare the length of the Array to the arity of the Method.

def Object.onearg(arg)
amethod = Object.method(:onearg)
aproc = proc{|arg| arg}

amethod and aproc both have arity of 1. they should generally behave the same. but, they behave differently when given as a block to Enumerable methods, with amethod behaving incorrectly when the Enumerable in question contains an array:

[[1, 2]].detect(&amethod)
ArgumentError: wrong number of arguments (2 for 1)

this seems to incorrectly compare the length of the element [1, 2] to the arity of amethod, even though it's passing one argument (the array [1, 2]) to amethod.

the Proc behaves correctly:

[[1, 2]].detect(&aproc)
=> [1, 2]

this does not compare the element's length to the arity of aproc, and so works fine.

Giving the Method as a block works fine when the arity happens to be the same as the length of the element which is an array:

=> [1]

Even though it is passed the whole array (seen in the return value), and not the element of the array.

File is attached to minimally reproduce, and its output is:

[1, 2]
methodarity.rb:10:in onearg': wrong number of arguments (2 for 1) (ArgumentError)
from methodarity.rb:10:in
from methodarity.rb:7:in detect'
from methodarity.rb:10:in
from methodarity.rb:10:in `detect'
from methodarity.rb:10

Tested on:
ruby 1.8.6 (2009-06-08 patchlevel 369) [universal-darwin9.0]
ruby 1.8.6 (2010-02-05 patchlevel 399) [i686-darwin9.8.0]
ruby 1.8.7 (2009-06-12 patchlevel 174) [i686-darwin9.8.0]
ruby 1.8.7 (2010-06-23 patchlevel 299) [i686-darwin9.8.0]
ruby 1.8.6 (2007-09-24 patchlevel 111) [i386-mswin32]
ruby 1.8.6 (2010-02-04 patchlevel 398) [i386-mingw32]
ruby 1.8.7 (2010-01-10 patchlevel 249) [i386-mingw32]

Behaves correctly in ruby 1.9.*.


methodarity.rb (347 Bytes) methodarity.rb ruby script to minimally reproduce the incorrect behavior Ethan (Ethan -), 10/13/2010 03:19 AM

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