In SO question 2068165 one answer raised the idea of using something like this:
params[:task][:completed_at] &&= Time.parse(params[:task][:completed_at])
as a DRYer way of saying
params[:task][:completed_at] = Time.parse(params[:task][:completed_at]) if params[:task][:completed_at]
where the params Hash would be coming from a (Rails/ActionView) form.
It's a kind of corollary to the well-known ||= idiom, setting the value if the LHS is not nil/false.
Is using &&= like this actually a recognised Ruby idiom that I've somehow missed or have I just forgotten a more commonly-used idiom? It is getting rather late...
It ought to be. If nothing else, params[:task] is only evaluated once when using the &&= form.
To clarify:
params[:task][:completed_at] = params[:task][:completed_at] && ...
calls [](:task) on params twice, [](:completed_at) and []=(:completed_at) once each on params[:task].
params[:task][:completed_at] &&= ...
calls [](:task) on params once, and its value is stashed away for both the [](:completed_at) and []=(:completed_at) calls.
Actual example describing what I'm trying to illustrate (based on Marc-Andre's example code; much thanks):
class X
def get
puts "get"
#hash ||= {}
end
end
irb(main):008:0> x = X.new
=> #<X:0x7f43c496b130>
irb(main):009:0> x.get
get
=> {}
irb(main):010:0> x.get[:foo] = 'foo'
get
=> "foo"
irb(main):011:0> x.get[:foo]
get
=> "foo"
irb(main):012:0> x.get[:foo] &&= 'bar'
get
=> "bar"
irb(main):013:0> x.get[:foo] = x.get[:foo] && 'bar'
get
get
=> "bar"
Note that using the "expanded" form causes "get" to be printed out twice, but using the compact form causes it to only be printed once.
Using &&=, in the case of LHS is false, it is only being read once, but not being set. This should make it clearer ...
class Test
def initialize(value)
#v = value
end
def v=(value)
puts "set"
#v = value
end
def v
puts "get=>#{#v}"
#v
end
end
t = Test.new(true)
t.v = t.v && true
puts '----'
t.v &&= true
puts '----'
t = Test.new(false) # lets make LHS false
t.v = t.v && true
puts '----'
t = Test.new(false) # lets make LHS false
t.v &&= true
The result:
get=>true
set
----
get=>true
set
----
get=>false
set
----
get=>false
Related
This question already has answers here:
What does ||= (or-equals) mean in Ruby?
(23 answers)
Closed 8 years ago.
I'm looking over someone's codes and wondering about the difference between:
def blah
#hello ||= [1,2,3].collect{|x| x+1}
end
and
def blah
#hello = [1,2,3].collect{|x| x+1}
end
I understand that ||= means "or equal", but why do we need it? An example would be great.
Also, for the collect method, let's say I have an array:
a = [1,2,4,5]
and I wanted to find the array that contains integers that are greater than 2, how can I use collect with that?
a.collect{|x| x>2} # => [false, false, true, true]
I want [4,5].
||= does nothing if the variable has been assigned a value (other than false or nil):
irb(main):001:0> blah ||= 'foo'
=> "foo"
irb(main):002:0> blah ||= 'bar'
=> "foo"
irb(main):003:0> puts blah
foo
=> nil
irb(main):004:0> blah = nil
=> nil
irb(main):005:0> blah ||= 'bar'
=> "bar"
irb(main):006:0> puts blah
bar
=> nil
One possible use is to assign variables in a fault-tolerant way. Compare:
#data = get_data_from_some_source
#data ||= get_data_from_fallback_source
#data ||= get_data_from_last_resort_source
with:
#data = get_data_from_some_souce
if #data == nil
#data = get_data_from_fallback_source
end
if #data == nil
#data = get_data_from_last_resort_source
end
or even (saints preserve us):
begin
#data = get_data_from_some_source
rescue GettingDataDidntWorkException
begin
#data = get_data_from_fallback_source
rescue GettingDataDidntWorkException
#data = get_data_from_last_resort_source
end
end
Haskell programmers will recognize this as similar in use to the Maybe monad.
a = [1,2,4,5] and I wanted to find the array that contains integers that are greater than 2,
Here we go using Array#select:
a.select{|e| e > 2 } # => [4,5]
Explanation of the part #hello ||= [1,2,3].collect{|x| x+1} can be found What does ||= (or equals) mean in Ruby?
The page What does ||= (or-equals) mean in Ruby? does not mention the "proxy design pattern", which it is in a nutshell.
The first time you call that line, the .select stuff gets evaluated. The second time, the system re-uses the #hello value, without re-assigning it. Due to "boolean short-circuiting", things to the right of a || don't evaluate, if the left side is not false or nil.
(Also, if you never evaluated #hello before, Ruby automagically creates one, and assigns nil to it. So Ruby lets you proxy in one line, as an optimization.)
Ok, so ||= means if you already have a previous value for your variable it will keep it
else, if it doesn't have any pre-assigned value, then it will take whatever value you set it equal to.
EX:
#hello = [1]
def blah
#hello ||=[1,2,3]
end
blah
#hello => [1], not [1,2,3]
Using Ruby 1.9.2
Problem
Compare the content, not the results, of two procs. I understand the results can't be tested because of the halting problem but that's OK; I don't want to test the results anyway.
For instance
proc {#x == "x"} == proc {#x == "x"} => false # doh!
That returns false because the objects inside the procs are not the same.
My clunky solution
I have a work around solution that kinda sorta does what I want but it doesn't really test that the proc is "equal" to what I put in it. In my specific case the format of my procs will always be boolean tests on instance variables like this:
{#x == "x" && #y != "y" || #z == String}
I wrote a method that builds classes dynamically and creates instance variables set to specified values:
def create_proc_tester(property_value_hash)
new_class = Class.new.new
new_class.class.class_eval do
define_method(:xql?) { |&block| instance_eval &block }
end
property_value_hash.each do |key, value|
new_class.instance_variable_set("##{key}", value)
end
new_class
end
Which could be used something like this:
class Foo
attr_accessor :block
end
foo = Foo.new
foo.block = proc {#x == "x" && #y != "y" || #z == String}
tester = create_proc_tester(:x => "x", :y => "y", :z => Fixnum)
puts "Test #1: #{tester.xql? &foo.block}"
tester = create_proc_tester(:x => "x", :y => "x", :z => String)
puts "Test #2: #{tester.xql? &foo.block}"
> Test #1: false
> Test #2: true
.
.
That's all great and wonderful but I want to know if there is a better, more meta, way to do this that actually tests the contents of the proc not just a work around that solves my specific problem; something that could be used to test any proc.
I was thinking there might be a way to use the Ruby parser to get something to compare but I have no idea how. I'm researching it now but thought I'd try to see if anyone here has done this before and knows how. That might be a dead-end though because of the dynamic nature of Ruby but that's where I'm looking now.
If you're using Ruby 1.9, you may be able to use the sourcify gem.
$ irb
> require 'sourcify'
=> true
> a = proc {#x == "x"}
=> #<Proc:0x9ba4240#(irb):2>
> b = proc {#x == %{x}}
=> #<Proc:0x9ba23f0#(irb):3>
> a == b
=> false
> a.to_source == b.to_source
=> true
> RUBY_VERSION
=> "1.9.2"
We also ran into the ParseTree/Ruby 1.9 incompatibility problem at my company.
$ sudo gem install ruby2ruby ParseTree
require 'parse_tree'
require 'ruby2ruby'
require 'parse_tree_extensions'
# All of these are the same:
proc { puts 'a' }.to_ruby # => "proc { puts(\"a\") }"
lambda { puts "a" }.to_ruby # => "proc { puts(\"a\") }"
Proc.new { puts %{a} }.to_ruby # => "proc { puts(\"a\") }"
# If you need to do this with classes:
class Bar; define_method(:foo) { 'a' }; end
puts Ruby2Ruby.new.process(Unifier.new.process(ParseTree.translate(Bar)))
# will print this:
# class Bar < Object
# def foo
# "a"
# end
# end
I have a question regarding the ||= statement in ruby and this is of particular interest to me as I'm using it to write to memcache. What I'm wondering is, does ||= check the receiver first to see if it's set before calling that setter, or is it literally an alias to x = x || y
This wouldn't really matter in the case of a normal variable but using something like:
CACHE[:some_key] ||= "Some String"
could possibly do a memcache write which is more expensive than a simple variable set. I couldn't find anything about ||= in the ruby api oddly enough so I haven't been able to answer this myself.
Of course I know that:
CACHE[:some_key] = "Some String" if CACHE[:some_key].nil?
would achieve this, I'm just looking for the most terse syntax.
This is extremely easy to test:
class MyCache
def initialize
#hash = {}
end
def []=(key, value)
puts "Cache key '#{key}' written"
#hash[key] = value
end
def [](key)
puts "Cache key '#{key}' read"
#hash[key]
end
end
Now simply try the ||= syntax:
cache = MyCache.new
cache["my key"] ||= "my value" # cache value was nil (unset)
# Cache key 'my key' read
# Cache key 'my key' written
cache["my key"] ||= "my value" # cache value is already set
# Cache key 'my key' read
So we can conclude that no assignment takes place if the cache key already exists.
The following extract from the Rubyspec shows that this is by design and should not be dependent on the Ruby implementation:
describe "Conditional operator assignment 'obj.meth op= expr'" do
# ...
it "may not assign at all, depending on the truthiness of lhs" do
m = mock("object")
m.should_receive(:foo).and_return(:truthy)
m.should_not_receive(:foo=)
m.foo ||= 42
m.should_receive(:bar).and_return(false)
m.should_not_receive(:bar=)
m.bar &&= 42
end
# ...
end
In the same file, there is a similar spec for [] and []= that mandates identical behaviour.
Although the Rubyspec is still a work in progress, it has become clear that the major Ruby implementation projects intend to comply with it.
According to §11.3.1.2.2 of the Draft ISO Specification,
CACHE[:some_key] ||= "Some String"
expands to
o = CACHE
*l = :some_key
v = o.[](*l)
w = "Some String"
x = v || w
l << x
o.[]=(*l)
x
Or, in the more general case
primary_expression[indexing_argument_list] ω= expression
(I am using ω here to denote any operator, so it could be ||=, +=, *=, >>=, %=,…)
Expands to:
o = primary_expression
*l = indexing_argument_list
v = o.[](*l)
w = expression
x = v ω w
l << x
o.[]=(*l)
x
So, according to the specification, []= will always get called. But that is actually not the case in current implementations (I tested MRI, YARV, Rubinius, JRuby and IronRuby):
def (h = {}).[]=(k, v) p "Setting #{k} to #{v}"; super end
h[:key] ||= :value # => :value
# "Setting key to value"
h[:key] ||= :value # => :value
So, obviously either the specification is wrong or all five currently released implementations are wrong. And since the purpose of the specification is to describe the behavior of the existing implementations, it's obviously that the specification must be wrong.
In general, as a first approximation
a ||= b
expands to
a || a = b
However, there's all kinds of subleties involved, for example, whether or not a is undefined, whether a is a simple variable or a more complex expression like foo[bar] or foo.bar and so on.
See also some of the other instances of this same question, that have already been asked and answered here on StackOverflow (for example, this one). Also, the question has been discussed so many times on the ruby-talk mailinglist, that there are now discussion threads whose sole purpose it is to summarize the other discussion threads. (Although please note that that list is far from complete.)
Here's another demonstration that's a bit different than the other answers in that it explicitly shows when the Hash is being written to:
class MyHash < Hash
def []=(key, value)
puts "Setting #{key} = #{value}"
super(key, value)
end
end
>> h = MyHash.new
=> {}
>> h[:foo] = :bar
Setting foo = bar
=> :bar
>> h[:bar] ||= :baz
Setting bar = baz
=> :baz
>> h[:bar] ||= :quux
=> :baz
And by way of comparison:
// continued from above
>> h[:bar] = h[:bar] || :quuux
Setting bar = baz
=> :baz
CACHE[:some_key] ||= "Some String"
is equivalent to
CACHE[:some_key] = "Some String" unless CACHE[:some_key]
(which is equivalent to if + nil? unless CACHE[:some_key] is a boolean value).
In other words: yes, ||= will only write if the LHS is nil or false.
[I removed my example that was less accurate than other's. I leave my answer for the benchmarks that might be interesting to some. My point was:]
So basically
CACHE[:some_key] ||= "Some String"
is the same as
CACHE[:some_key] = "Some String" unless CACHE[:some_key]
I'm more for the first syntax, but then it's up to you since readibility is a bit reduced in that case.
I was curious, so here's some benchmarks:
require "benchmark"
CACHE = {}
Benchmark.bm do |x|
x.report {
for i in 0..100000
CACHE[:some_key] ||= "Some String"
end
}
x.report {
for i in 0..100000
CACHE[:some_key] = "Some String" unless CACHE[:some_key]
end
}
end
user system total real
0.030000 0.000000 0.030000 ( 0.025167)
0.020000 0.000000 0.020000 ( 0.026670)
Is there a good way to chain methods conditionally in Ruby?
What I want to do functionally is
if a && b && c
my_object.some_method_because_of_a.some_method_because_of_b.some_method_because_of_c
elsif a && b && !c
my_object.some_method_because_of_a.some_method_because_of_b
elsif a && !b && c
my_object.some_method_because_of_a.some_method_because_of_c
etc...
So depending on a number of conditions I want to work out what methods to call in the method chain.
So far my best attempt to do this in a "good way" is to conditionally build the string of methods, and use eval, but surely there is a better, more ruby, way?
You could put your methods into an array and then execute everything in this array
l= []
l << :method_a if a
l << :method_b if b
l << :method_c if c
l.inject(object) { |obj, method| obj.send(method) }
Object#send executes the method with the given name. Enumerable#inject iterates over the array, while giving the block the last returned value and the current array item.
If you want your method to take arguments you could also do it this way
l= []
l << [:method_a, arg_a1, arg_a2] if a
l << [:method_b, arg_b1] if b
l << [:method_c, arg_c1, arg_c2, arg_c3] if c
l.inject(object) { |obj, method_and_args| obj.send(*method_and_args) }
You can use tap:
my_object.tap{|o|o.method_a if a}.tap{|o|o.method_b if b}.tap{|o|o.method_c if c}
Sample class to demonstrate chaining methods that return a copied instance without modifying the caller.
This might be a lib required by your app.
class Foo
attr_accessor :field
def initialize
#field=[]
end
def dup
# Note: objects in #field aren't dup'ed!
super.tap{|e| e.field=e.field.dup }
end
def a
dup.tap{|e| e.field << :a }
end
def b
dup.tap{|e| e.field << :b }
end
def c
dup.tap{|e| e.field << :c }
end
end
monkeypatch: this is what you want to add to your app to enable conditional chaining
class Object
# passes self to block and returns result of block.
# More cumbersome to call than #chain_if, but useful if you want to put
# complex conditions in the block, or call a different method when your cond is false.
def chain_block(&block)
yield self
end
# passes self to block
# bool:
# if false, returns caller without executing block.
# if true, return result of block.
# Useful if your condition is simple, and you want to merely pass along the previous caller in the chain if false.
def chain_if(bool, &block)
bool ? yield(self) : self
end
end
Sample usage
# sample usage: chain_block
>> cond_a, cond_b, cond_c = true, false, true
>> f.chain_block{|e| cond_a ? e.a : e }.chain_block{|e| cond_b ? e.b : e }.chain_block{|e| cond_c ? e.c : e }
=> #<Foo:0x007fe71027ab60 #field=[:a, :c]>
# sample usage: chain_if
>> cond_a, cond_b, cond_c = false, true, false
>> f.chain_if(cond_a, &:a).chain_if(cond_b, &:b).chain_if(cond_c, &:c)
=> #<Foo:0x007fe7106a7e90 #field=[:b]>
# The chain_if call can also allow args
>> obj.chain_if(cond) {|e| e.argified_method(args) }
Although the inject method is perfectly valid, that kind of Enumerable use does confuse people and suffers from the limitation of not being able to pass arbitrary parameters.
A pattern like this may be better for this application:
object = my_object
if (a)
object = object.method_a(:arg_a)
end
if (b)
object = object.method_b
end
if (c)
object = object.method_c('arg_c1', 'arg_c2')
end
I've found this to be useful when using named scopes. For instance:
scope = Person
if (params[:filter_by_age])
scope = scope.in_age_group(params[:filter_by_age])
end
if (params[:country])
scope = scope.in_country(params[:country])
end
# Usually a will_paginate-type call is made here, too
#people = scope.all
Use #yield_self or, since Ruby 2.6, #then!
my_object.
then{ |o| a ? o.some_method_because_of_a : o }.
then{ |o| b ? o.some_method_because_of_b : o }.
then{ |o| c ? o.some_method_because_of_c : o }
Here's a more functional programming way.
Use break in order to get tap() to return the result. (tap is in only in rails as is mentioned in the other answer)
'hey'.tap{ |x| x + " what's" if true }
.tap{ |x| x + "noooooo" if false }
.tap{ |x| x + ' up' if true }
# => "hey"
'hey'.tap{ |x| break x + " what's" if true }
.tap{ |x| break x + "noooooo" if false }
.tap{ |x| break x + ' up' if true }
# => "hey what's up"
Maybe your situation is more complicated than this, but why not:
my_object.method_a if a
my_object.method_b if b
my_object.method_c if c
I use this pattern:
class A
def some_method_because_of_a
...
return self
end
def some_method_because_of_b
...
return self
end
end
a = A.new
a.some_method_because_of_a().some_method_because_of_b()
If you're using Rails, you can use #try. Instead of
foo ? (foo.bar ? foo.bar.baz : nil) : nil
write:
foo.try(:bar).try(:baz)
or, with arguments:
foo.try(:bar, arg: 3).try(:baz)
Not defined in vanilla ruby, but it isn't a lot of code.
What I wouldn't give for CoffeeScript's ?. operator.
I ended up writing the following:
class Object
# A naïve Either implementation.
# Allows for chainable conditions.
# (a -> Bool), Symbol, Symbol, ...Any -> Any
def either(pred, left, right, *args)
cond = case pred
when Symbol
self.send(pred)
when Proc
pred.call
else
pred
end
if cond
self.send right, *args
else
self.send left
end
end
# The up-coming identity method...
def itself
self
end
end
a = []
# => []
a.either(:empty?, :itself, :push, 1)
# => [1]
a.either(:empty?, :itself, :push, 1)
# => [1]
a.either(true, :itself, :push, 2)
# => [1, 2]
Why isnt that working:
>> s = "hi"
=> "hi"
>> s == ("hi"|"ho")
NoMethodError: undefined method `|' for "hi":String
from (irb):2
>>
I don't get it.. Is there a solution for this kind of syntax? Because
s == ("hi"|"ho")
#is shorther than
s == "hi" || s == "ho"
Yes, the bitwise operator | is not defined in the String class: http://ruby-doc.org/core/classes/String.html
Consider this for expressiveness:
["hi", "ho"].include? myStr
irb(main):001:0> s = "hi"
=> "hi"
irb(main):002:0> ["hi", "ho"]
=> ["hi", "ho"]
irb(main):003:0> ["hi", "ho"].include? s
=> true
irb(main):004:0> s = "foo"
=> "foo"
irb(main):005:0> ["hi", "ho"].include? s
=> false
In most high level languages that syntax will not work, you have to stick to the longer syntax of:
s == "hi" || s == "ho"
Note that | is a bitwise or, whereas || is a regular or
You could use the include? method on array if you've got several == tests to do:
["hi", "ho"].include?(s)
Not shorter for two checks admittedly but it will be shorter for three or more.
This syntax doesn't exist in any language as far as I know.
What you are saying
s == ("hi"|"ho")
Literally translates to 'bitwise OR the strings "hi" and "ho" together and then compare them with s'. If you can't see why this is not what you are looking for, try writing down the ASCII codes for "hi" and "ho" and then bitwise ORing them together. You are going to get complete gibberish.
You could make it work that way:
irb> class Pair
def initialize(strA,strB)
#strA,#strB = strA,strB
end
def ==(string)
string == #strA || string == #strB
end
def |(other)
Pair.new(self,other)
end
end
#=> nil
irb> class String
def |(other)
Pair.new(self,other)
end
alias old_equals :==
def ==(other)
if other.kind_of? Pair
other == self
else
old_equals other
end
end
end
#=> nil
irb> ("one"|"two") == "one"
#=> true
irb> ("one"|"two") == "two"
#=> true
irb> ("one"|"two") == "three"
#=> false
irb> "one" == ("one"|"two")
#=> true
irb> "three" == ("one"|"two"|"three")
#=> true
But since this involves some monkey-patching of a fairly lowlevel class, I wouldn't advise relying on it. Other people will hate reading your code.
Ruby supports binary 'or' and other binary operations on values of type Fixnum and Bignum, meaning any integer. Bitwise operations aren't supported on strings or any other type, as far as I know.
As other people have mentioned, you probably want something other than binary operations altogether. However, you can easily get integer representations of characters, so you can compare characters like so:
a = "Cake"
b = "Pie"
puts a[0] | b[0] # Prints "83" - C is 67 and P is 80.
You can get an array of the comparisons easily with some conversions.
a = "Cake"
b = "Pie " # Strings of uneven length is trivial but more cluttered.
a_arr = a.split(//)
b_arr = b.split(//)
c_arr = []
a.each_with_index { |char, i| c.push(a[i].to_i | b[i].to_i) }
# If you *really* want an ASCII string back...
c = c_arr.collect(&:chr).join
You could use a regex:
Like so:
regex = /hi|ho/
s = "hi"
t = "foo"
s =~ regex
#=> 0
t =~ regex
#=> nil