Defining <<

Ruby
1

I’m not 100% how exactly to search to find out if someone else has posed
this question, but why does the following happen?

Given:

class Hash
def << (key, val=nil)
self.store(key, val)
end
end

h = {}
h << ‘test’
h << ‘test’, ‘bob’

Gives back:

test.rb:9: parse error, unexpected ‘,’, expecting $
h << ‘test’, ‘bob’

If I change the last line to:

h << (‘test’, ‘bob’)

I get back:

test.rb:9: parse error, unexpected ‘,’, expecting ‘)’
h << (‘test’, ‘bob’)

2

on occasion this space
def << (key, val=nil)

3

on occasion this space
def << (key, val=nil)
^

causes problems

4

On Feb 7, 2007, at 4:46 PM, Luke I. wrote:

end

h = {}
h << ‘test’
h << ‘test’, ‘bob’

The syntax rules for the << operator don’t allow it to take multiple
arguments when called via infix notation:

h << arg1 # one argument only

you can call the method with multiple arguments but you’ve got to do
it like:

h.<<(arg1, arg2) # dot-style method invocation

You can use an array to ‘cheat’:

h << [arg1, arg2]

But the method will only see one argument, an array, and you would have
to expect that and/or test for it in your definition for Hash#<<.

Gary W.

5

Robert K. wrote:

[…]
irb(main):013:0> h << “foo” << “bar” << “key” << 234
[…]

Not that I would recommend it…

Yeah, i like

h <= “foo” > “bar” <= “key” > 42

:slight_smile:

unfortunately there is no => operator…

cheers

Simon

6

On 07.02.2007 22:58, Gary W. wrote:

self.store(key, val)

h << arg1 # one argument only
But the method will only see one argument, an array, and you would have
to expect that and/or test for it in your definition for Hash#<<.

Here’s another variant:

class Hash
Proxy = Struct.new :parent, :key do
def <<(val)
parent[key] = val
parent
end
end
def <<(key)
Proxy.new self, key
end
end

irb(main):012:0> h ={}
=> {}
irb(main):013:0> h << “foo” << “bar” << “key” << 234
=> {“foo”=>“bar”, “key”=>234}
irb(main):014:0> h << 2 << 3
=> {2=>3, “foo”=>“bar”, “key”=>234}
irb(main):015:0> h
=> {2=>3, “foo”=>“bar”, “key”=>234}

Not that I would recommend it…

Kind regards

robert