Conditionally call chained method - ruby

A chained method should only be called under certain circumstances in the following code.
class Klass
def foo
puts 'foo'
self
end
def bar
puts 'bar'
self
end
end
klass = Klass.new
a = 2
id = klass.foo{conditionally chain bar if a == 2}.bar
Can you insert an expression or method between chained methods that conditionally continues or halts the method chain?

This is simple and who will come after you will understand immediately:
klass = klass.foo
klass = klass.bar if a == 2
etc...
This works well if the chained methods take no arguments
klass.define_singleton_method :chain_if do |b, *s|
return unless b
klass = self
s.each do |x|
klass = klass.send x
end
klass
end
klass.foo.chain_if(true, :foo, :bar).chain_if(false, :bar)
Here some duplicated threads!
conditional chaining in ruby
Add method to an instanced object
Here I found another solution that I personally like:
my_object.tap{|o|o.method_a if a}.tap{|o|o.method_b if b}.tap{|o|o.method_c if c}
EDIT:
beware tap is defined as follows:
class Object
def tap
yield self
self
end
end
What you need might look like this, if the chained method returns a new immutable object:
class Object
def tap_and_chain
yield self
end
end

def chain_maybe(klass, condition, *args)
args[1..-1].reduce(klass.send(args.first)) { |r,m| (condition && r.send(m)) || r }
end
or:
def chain_maybe(klass, condition, *args)
first, *others = args
others = [] unless condition
others.reduce(klass.send(first)) { |r,m| r.send(m) }
end
For:
class Klass
def foo
puts 'foo'
self
end
def bar
puts 'bar'
self
end
def baz
puts 'baz'
self
end
end
c = Klass.new
chain_maybe(c, true, :foo, :bar, :baz)
foo
bar
baz
#=> #<Klass:0x007fccea8da388>
chain_maybe(c, false, :foo, :bar, :baz)
foo
#=> #<Klass:0x007fccea8da388>
chain_maybe(c, true, :foo, :bar)
foo
bar
#=> #<Klass:0x007fccea8da388>
chain_maybe(c, true, :bar, :baz)
bar
baz
#=> #<Klass:0x007fccea8da388>
If there is to be a condition for each argument, this can be generalized to:
def chain_maybe(klass, conditions, args)
return nil if conditions.empty?
first, *others = args.zip(conditions).select(&:last).map(&:first)
others.reduce(klass.send(first)) { |r,m| r.send(m) }
end
args = [:foo, :bar, :baz]
chain_maybe(c, [true, true, true], args)
foo
bar
baz
#=> #<Klass:0x007fccea8da388>
chain_maybe(c, [false, true, true], args)
bar
baz
#=> #<Klass:0x007fccea8da388>
chain_maybe(c, [true, false, true], args)
foo
baz
#=> #<Klass:0x007fccea8da388>

You can use tap if the block you are chaining is mutable, i.e: It will change the value of self (as explained here)
However, if you are chaining immutable blocks like QueryMethods, you can use try like:
data
.try { |d| group ? d.group(group) : d }
.try { |d| group ? d.order(order => :desc) : d }
try with nothing else but a block is essentially:
def try
yield self
end
(reference: #try)

Related

How to remove or bypass a superclass method in Ruby?

Some background: I have an external library that uses explicit type checking instead of duck-typing in one of its methods. Something like:
def a_method(value)
case value
when Array then 'an Array'
when Hash then 'a Hash'
when Foo then 'a Foo'
end
end
Foo is defined in the library. I would like to pass another object to this method that should be treated like a Foo. Therefore, I'm subclassing Foo:
class Bar < Foo
end
which works just fine:
bar = Bar.new
a_method(bar)
#=> 'a Foo'
Unfortunately, Foo implements several methods that will break the way Bar is supposed to work, including method_missing and respond_to?. For example:
class Foo
def respond_to?(method_name)
false
end
end
Because Foo is Bar's superclass, Foo#respond_to? is invoked when calling Bar#respond_to?:
class Bar < Foo
def hello
end
end
bar = Bar.new
bar.respond_to?(:hello) #=> false
bar.method(:respond_to?) #=> #<Method: Bar(Foo)#respond_to?>
I would like to remove or bypass Foo's method in this case (i.e. from within Bar) so that:
bar.respond_to?(:hello) #=> true
bar.method(:respond_to?) #=> #<Method: Bar(Kernel)#respond_to?>
just as if Foo#respond_to? did not exist.
Any suggestions?
Suppose you had this class structure:
class A
def respond_to?(meth)
"A"
end
def cat
end
def tap
puts "tap in A"
end
end
class B < A
def respond_to?(meth)
"B"
end
end
class C < B
def respond_to?(meth)
"C"
end
end
class D < C
end
We have:
D.ancestors
#=> [D, C, B, A, Object, Kernel, BasicObject]
Further, you want:
D.new.respond_to?(:cat)
#=> true
If we write:
class D < C
def respond_to?(meth)
method(__method__).super_method.call(meth)
end
end
then:
D.new.respond_to?(:cat)
#=> C
This is not surprising, since it's the same as:
class D < C
def respond_to?(meth)
super
end
end
D.new.respond_to?(:cat)
#=> C
Now try:
class D < C
def respond_to?(meth)
method(__method__).super_method.super_method.call(meth)
end
end
then:
D.new.respond_to?(:cat)
#=> B
so we have skipped over C. Now redefine D#respond_to? as follows:
class D < C
def respond_to?(meth)
method(__method__).super_method.super_method.super_method.call(meth)
end
end
D.new.respond_to?(:cat)
#=> A
Once more:
class D < C
def respond_to?(meth)
method(__method__).super_method.super_method.super_method.super_method.call(meth)
end
end
D.new.respond_to?(:cat)
#=> True (invokes Kernel#respond_to?)
Therefore, you could do the following:
module JumpOver
def jump_over(over_mod, meth)
m = instance_method(meth)
loop do
m = m.super_method
break if m.owner > over_mod
end
define_method(meth, m)
end
end
class D
extend JumpOver
jump_over(A, :respond_to?)
jump_over(A, :tap)
end
D.methods.include?(:jump)
#=> true
D.instance_methods(false)
#=> [:respond_to?, :tap]
d = D.new
#=> #<D:0x007ff263161b58>
d.respond_to? :cat
#=> true
d.respond_to? :dog
#=> false
d.tap { |o| puts 'hi' }
# 'hi'
#=> #<D:0x007ff263161b58>
I don't understand the problem. Just implement a respond_to? in Bar and don't call super.
class Foo
def respond_to?(method)
puts 'in foo'
false
end
end
class Bar < Foo
def respond_to?(method)
puts 'in bar'
true
end
end
bar = Bar.new
bar.respond_to?(:quux) # => true
# >> in bar
This is, of course, a violation of LSP, but you specifically asked for it, so... :)
Ruby's method lookup seems to be "hard-coded". I couldn't find a way to alter it from within Ruby.
Based on Sergio Tulentsev's suggestion to re-implement the methods, however, I came up with a helper method to replace / overwrite every superclass (instance) method with its "super" method (or undefine it if there is none):
def self.revert_superclass_methods
superclass.instance_methods(false).each do |method|
super_method = instance_method(method).super_method
if super_method
puts "reverting #{self}##{method} to #{super_method}"
define_method(method, super_method)
else
puts "undefining #{self}##{method}"
undef_method(method)
end
end
end
This has basically the same effect for my purposes. Example usage:
module M
def foo ; 'M#foo' ; end
end
class A
include M
def to_s ; 'A#to_s' ; end
def foo ; 'A#foo' ; end
def bar ; 'A#bar' ; end
end
class B < A
def self.revert_superclass_methods
# ...
end
end
b = B.new
b.to_s #=> "A#to_s"
b.foo #=> "A#foo"
b.bar #=> "A#bar"
B.revert_superclass_methods
# reverting B#to_s to #<UnboundMethod: Object(Kernel)#to_s>
# reverting B#foo to #<UnboundMethod: Object(M)#foo>
# undefining B#bar
b.to_s #=> "#<B:0x007fb389987490>"
b.foo #=> "M#foo"
b.bar #=> undefined method `bar' for #<B:0x007fb389987490> (NoMethodError)

How to initialize class in block in Ruby?

I can't figure out the proper block initialize
class Foo
attr_accessor :bar
end
obj = Foo.new do |a|
a.bar = "baz"
end
puts obj.bar
Expect "baz"
instead get nil
What is the proper incantation for block class initializers in ruby?
Another way to make a block initializer would be writing it yourself one:
class Foo
attr_accessor :bar
def initialize
yield self if block_given?
end
end
And later use it:
foo = Foo.new do |f|
f.bar = true
end
My two cents.
Try again:
class Foo
attr_accessor :bar
end
obj = Foo.new.tap do |a|
a.bar = "baz"
end
puts obj.bar
I don't think new can take a block. Never saw it anywhere anyway. Why do you want to initialize in a block ? You can always do obj = foo.new.tap do |a| ... If you really want a block
actually you have a constructor for these purposes:
class Foo
attr_accessor :bar
def initialize(bar = "baz")
#bar = bar
end
end

Turning constructor arguments into instance variables [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Idiomatic object creation in ruby
There are many occaisions when I have an initialize method that looks like this:
class Foo
def initialize bar, buz, ...
#bar, #buz, ... = bar, buz, ...
end
end
Is there a way to do this with a simple command like:
class Foo
attr_constructor :bar, :buz, ...
end
where the symbols represent the name of the instance variables (with the spirit/flavor of attr_accessor, attr_reader, attr_writer)?
I was wondering if there is a built in way or a more elegant way of doing something like this:
class Class
def attr_constructor *vars
define_method("initialize") do |*vals|
vars.zip(vals){|var, val| instance_variable_set("##{var}", val)}
end
end
end
so that I can use it like this:
class Foo
attr_constructor :foo, :bar, :buz
end
p Foo.new('a', 'b', 'c') # => #<Foo:0x93f3e4c #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b', 'c', 'd') # => #<Foo:0x93f3e4d #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b') # => #<Foo:0x93f3e4e #foo="a", #bar="b", #buz=nil>
I'd use OpenStruct:
require 'ostruct'
class Foo < OpenStruct
end
f = Foo.new(:bar => "baz")
f.bar
#=> "baz"
Edit: Ah OK, sorry misunderstood you. How about just:
class Foo
def initialize(*args)
#baz, #buz = args
end
end
Would this work for you?
class Foo
def initialize(hash)
hash.each { |k,v| instance_variable_set("##{k}", v) }
end
end
Interesting question. A little meta-programming should take care of it.
module Attrs
def self.included(base)
base.extend ClassMethods
base.class_eval do
class << self
attr_accessor :attrs
end
end
end
module ClassMethods
# Define the attributes that each instance of the class should have
def has_attrs(*attrs)
self.attrs = attrs
attr_accessor *attrs
end
end
def initialize(*args)
raise ArgumentError, "You passed too many arguments!" if args.size > self.class.attrs.size
# Loop through each arg, assigning it to the appropriate attribute (based on the order)
args.each_with_index do |val, i|
attr = self.class.attrs[i]
instance_variable_set "##{attr}", val
end
end
end
class Foo
include Attrs
has_attrs :bar, :buz
end
f = Foo.new('One', 'Two')
puts f.bar
puts f.buz
Of course the downside to this is inflexibility - you have to pass your constructor arguments in a specific order. Of course that's how most programming languages are. Rails people might argue you should instead do
f = Foo.new(:bar => 'One', :baz => 'Two')
which would allow you to pass in attrs in any order, as well as strip away most of the meta-programming. But that is a lot more to type.

Custom << method

class Foo
def initialize
#bar = []
end
def changed_callback
puts "Bar has been changed!"
end
def bar
#bar
end
def bar=(a)
#bar = a
self.changed_callback() # (hence why this doesn't just use attr_accessor)
end
def bar<<(a)
#bar.push(a)
self.changed_callback()
end
end
f = Foo.new()
f.bar = [1,2,3]
=> "Bar has been changed!"
f.bar << 4
=> "Bar has been changed!"
puts f.bar.inspect
=> [1,2,3,4]
Is anything like that possible?
Thanks!
You need to somehow extend the object returned by Foo#bar with an appropriate #<< method. Something like this, maybe?
class Foo
module ArrayProxy
def <<(other)
#__foo__.changed_callback
super
end
end
def initialize
#bar = []
end
def changed_callback
puts 'Bar has been changed!'
end
def bar
return #bar if #bar.is_a?(ArrayProxy)
#bar.tap {|bar| bar.extend(ArrayProxy).instance_variable_set(:#__foo__, self) }
end
def bar=(a)
#bar = a
changed_callback # (hence why this doesn't just use attr_accessor)
end
end
f = Foo.new
f.bar = [1,2,3]
# "Bar has been changed!"
f.bar << 4
# "Bar has been changed!"
puts f.bar.inspect
# => [1,2,3,4]

Is 'yield self' the same as instance_eval?

Is there any difference if you define Foo with instance_eval: . . .
class Foo
def initialize(&block)
instance_eval(&block) if block_given?
end
end
. . . or with 'yield self':
class Foo
def initialize
yield self if block_given?
end
end
In either case you can do this:
x = Foo.new { def foo; 'foo'; end }
x.foo
So 'yield self' means that the block after Foo.new is always evaluated in the context of the Foo class.
Is this correct?
Your two pieces of code do very different things. By using instance_eval you're evaluating the block in the context of your object. This means that using def will define methods on that object. It also means that calling a method without a receiver inside the block will call it on your object.
When yielding self you're passing self as an argument to the block, but since your block doesn't take any arguments, it is simply ignored. So in this case yielding self does the same thing as yielding nothing. The def here behaves exactly like a def outside the block would, yielding self does not actually change what you define the method on. What you could do is:
class Foo
def initialize
yield self if block_given?
end
end
x = Foo.new {|obj| def obj.foo() 'foo' end}
x.foo
The difference to instance_eval being that you have to specify the receiver explicitly.
Edit to clarify:
In the version with yield, obj in the block will be the object that is yielded, which in this case is is the newly created Foo instance. While self will have the same value it had outside the block. With the instance_eval version self inside the block will be the newly created Foo instance.
They are different. yield(self) does not change the value of self inside the block, while instance_eval(&block) does.
class Foo
def with_yield
yield(self)
end
def with_instance_eval(&block)
instance_eval(&block)
end
end
f = Foo.new
f.with_yield do |arg|
p self
# => main
p arg
# => #<Foo:0x100124b10>
end
f.with_instance_eval do |arg|
p self
# => #<Foo:0x100124b10>
p arg
# => #<Foo:0x100124b10>
end
You just can drop the self keyword
class Foo
def initialize
yield if block_given?
end
end
Update from comments
Using yield there is a bit new to my taste, specially when used outside irb.
However there is a big and significant difference between instance_eval approach and yield approach, check this snippet:
class Foo
def initialize(&block)
instance_eval(&block) if block_given?
end
end
x = Foo.new { def foo; 'foo'; end }
#=> #<Foo:0xb800f6a0>
x.foo #=> "foo"
z = Foo.new #=> #<Foo:0xb800806c>
z.foo #=>NoMethodError: undefined method `foo' for #<Foo:0xb800806c>
Check this one as well:
class Foo2
def initialize
yield if block_given?
end
end
x = Foo2.new { def foo; 'foo'; end } #=> #<Foo:0xb7ff1bb4>
x.foo #=> private method `foo' called for #<Foo2:0xb8004930> (NoMethodError)
x.send :foo => "foo"
z = Foo.new #=> #<Foo:0xb800806c>
z.send :foo => "foo"
As you can see the difference is that the former one is adding a singleton method foo to the object being initialized, while the later is adding a private method to all instances of Object class.

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