Ruby for in loop error with { } block delimiter

Question

In the language Ruby the following works in irb

for fruit in ['apple', 'banana', 'cherry', 'date'] do
    puts fruit 
end

but this one does not

# error
for fruit in ['apple', 'banana', 'cherry', 'date'] { puts fruit }

please note for reference the following block delimiters do not error

5.times do |i|
    puts "hello "+i.to_s
end

5.times { |i| puts "hello "+i.to_s }

Edit: I guess what I'm observing is an inconsistency with the way do end is substituted for { } can someone explain why or please point out my mistake?

don't know why, but there is a similar restriction in refine <class> do as well. — max pleaner
– max pleaner, Commented Dec 29, 2016 at 5:16
If it's any consolation, you don't need the opening do in a for loop. — Sagar Pandya
– Sagar Pandya, Commented Dec 29, 2016 at 5:31
This might be to do with the fact that for and while (which acts like for) are not methods. But I await a formal reasoning. — Sagar Pandya
– Sagar Pandya, Commented Dec 29, 2016 at 5:43
This is a perfect example of why we should always avoid and consider being a code smell for and while loops in ruby. — Aleksei Matiushkin
– Aleksei Matiushkin, Commented Dec 29, 2016 at 6:56
I cannot find where for is defined in the official Ruby doc. Does somebody know where it is? — Eric Duminil
– Eric Duminil, Commented Dec 29, 2016 at 10:25

Community · Accepted Answer · 2017-05-23 12:24:33Z

10

The following quote from this post explains the "inconsistency" well (emphasis mine):

end is a generic keyword that ends a context (conditionals, class and module definitions, method definitions, loops and exception-catching [statements]). Curly braces are special syntax for defining blocks or Hashes.

for loops are control structures - not methods with a block argument - hence they must be closed with the end keyword. Methods such asEnumerable#times and Enumerable#each have a block argument which can be defined with either of the { } or do ... end syntaxes.

edited May 23, 2017 at 12:24

CommunityBot

11 silver badge

answered Dec 29, 2016 at 6:17

user1351104

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1 Comment

PrimeLens Over a year ago

great answer! Thanks.

Eric Duminil · Accepted Answer · 2016-12-29 10:39:15Z

0

for isn't a method defined for Object or in Kernel, and it doesn't expect a block.

You cannot write :

class A {
  def method {
  }
}

either.

You could define a custom my_for method :

module Kernel
  def my_for(elements, &block)
    elements.each(&block)
  end
end

my_for ['apple','banana','cherry', 'date'] do |fruit|
  puts fruit
end

my_for ['apple','banana','cherry', 'date'] { |fruit| puts fruit }

answered Dec 29, 2016 at 10:39

Eric Duminil

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1 Comment

user1351104 Over a year ago

for ... in is a valid control expression in Ruby, see the documentation.

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