I wanted to start using Steep for one of my projects, but I keep getting errors that I don't know how to fix. This is one example extracted from my code that I am struggling with. I removed all the complexity except for the relevant bits:
Setup
# lib/a/b.rb
module A
class B
C = [[''], ['2', '2\''], ['\'', '3', '’']].freeze
D = (['0'] + C.map(&:first)).freeze
def initialize(v)
#v = v
end
def <=>(other)
#v <=> other.v
end
end
end
My signatures look like this:
# sig/a/b.rbs
module A
class B
C: ::Array[::Array[::String]]
D: ::Array[::String]
#v: ::Integer
def initialize: (::Integer v) -> untyped
attr_reader v: ::Integer
def <=>: (self other) -> ::Integer
end
end
Steepfile:
target :lib do
signature 'sig'
check 'lib'
end
Steep now says:
lib/a/b.rb:4:4: IncompatibleAssignment: lhs_type=::Array[::String], rhs_type=::Array[(::String | nil)] (D = (['0'] + C.map(&:first)).freeze)
::Array[(::String | nil)] <: ::Array[::String]
(::String | nil) <: ::String
nil <: ::String
==> nil <: ::String does not hold
lib/a/b.rb:10:4: MethodBodyTypeMismatch: method=<=>, expected=::Integer, actual=(::Integer | nil) (def <=>(other))
(::Integer | nil) <: ::Integer
nil <: ::Integer
==> nil <: ::Integer does not hold
Problem
I think I understand the complaint. In general Array#first returns either a value from that array or nil. However, my Array was just defined one line above and so I know that it's not nil in this particular case.
Same thing for <=>. It could return nil in some cases, but given that I typed the argument other as self, I know other.value is an Integer and a <=> b where a and b are integers returns an integer. I.e. I know that it's not nil here.
But how do I tell steep that? Can I cast this to non-nil somehow?
Related
Say if you were using the built-in iterator .each method on a range:
(0..3).each do |x,y,z,a,b,c|
end
I know that x is the actual item in the collection, but what does defining additional block variables (y,z,a,b,c) to the block actually do? Do they represent something?
If the method does not give values, then the additional variables will be nil.
When running :
(0..3).each do |x,y|
puts x.class.to_s() + ' ' + y.class.to_s()
end
It outputs :
Integer NilClass
Integer NilClass
Integer NilClass
Integer NilClass
It all depends on what the method sends :
def myBlockCaller
yield 1, 2, 3
yield 'a', '5'
yield true
yield nil, 1
end
myBlockCaller do |x, y, z|
puts x.class.to_s() + ', ' + y.class.to_s() + ', ' + z.class.to_s()
end
Shows :
Integer, Integer, Integer
String, String, NilClass
TrueClass, NilClass, NilClass
NilClass, Integer, NilClass
They don't "do" anything. If you try it you will see they are nil.
(0..3).each do |x,y,z,a,b,c|
puts x.inspect
puts y.inspect
puts z.inspect
puts a.inspect
puts b.inspect
puts c.inspect
end
1
nil
nil
nil
nil
nil
2
nil
nil
nil
nil
nil
3
nil
nil
nil
nil
nil
Some iterators use more than one variable like each_with_index:
(1..3).each_with_index do |n,i|
n will hold the range item and i is it's index, i.e. 0,1,2
Or in a hash like:
{a: 1, b: 2, c: 3}.each do |k,v|
puts k
puts v
end
a:
1
b:
2
c:
3
def key_for_min_value(name_hash)
name_hash.max_by {|k, v| 0-v}[0]
end
This was my code to fulfill the test suite for finding the lowest value of a hash (this was for one of my lessons online).
I know there are much easier ways to do this but I had some restrictions, as you can see below:
**A Few Restrictions:
We want you to build this on your own. Some of the following methods are helpful but off limits for this exercise. (We'll cover a few below in more depth in subsequent lessons).
I could not use keys, values, min, sort, min_by to make it pass.
This code returned the key with the lowest value (a hash of key ==> integers) but here was the requirement I could not figure out.
If the method is called and passed an argument of an empty hash, it should return nil.
Only first month programming, so this may be obvious but is there a way to return nil for an empty hash, and keep my existing code intact?
Thanks for your help
To a beginner programmer I would recommend to print all intermediate results of expressions, or work in IRB.
def key_for_min_value(name_hash)
puts
puts "in key_for_min_value with parameter #{name_hash}"
# puts "about to return nil" if name_hash.empty?
# return nil if name_hash.empty?
name_hash.max_by { | item | puts "item=#{item}" }
max = name_hash.max_by do | k, v |
puts "k=#{k} v=#{v} 0 - v = #{0 - v}"
0 - v
end
puts "max=#{max.inspect}, class of value returned by max_by : #{max.class}"
result = name_hash.max_by {|k, v| 0-v}[0]
puts "result=#{result.inspect}"
result
end
key_for_min_value({a: 1, b: 2, c: 3})
key_for_min_value({})
Execution :
$ ruby -w t.rb
in key_for_min_value with parameter {:a=>1, :b=>2, :c=>3}
item=[:a, 1]
item=[:b, 2]
item=[:c, 3]
k=a v=1 0 - v = -1
k=b v=2 0 - v = -2
k=c v=3 0 - v = -3
max=[:a, 1], class of value returned by max_by : Array
result=:a
in key_for_min_value with parameter {}
max=nil, class of value returned by max_by : NilClass
t.rb:15:in `key_for_min_value': undefined method `[]' for nil:NilClass (NoMethodError)
from t.rb:21:in `<main>'
The documentation of enum.max_by says :
Returns the item corresponding to the largest value returned by the
block.
But if the enum is empty, it returns nil, from which you fetch element [0], which causes the error because there is no such method in the NilClass.
If you add return nil if name_hash.empty? at the beginning of the method, you prevent it to happen (with two uncommented lines) :
$ ruby -w t.rb
in key_for_min_value with parameter {:a=>1, :b=>2, :c=>3}
...
in key_for_min_value with parameter {}
about to return nil
There a lot of different possibilities to do what you want. The most obvious one is to literally translate the sentence: "return nil if the hash is empty" into Ruby:
def key_for_min_value(name_hash)
return nil if name_hash.empty?
name_hash.max_by {|k, v| 0-v}[0]
end
Another possibility would be to use the safe navigation operator:
def key_for_min_value(name_hash)
name_hash.max_by {|k, v| 0-v}&.[](0)
end
Yet another way would be to ensure that the value you are trying to index into is never nil:
def key_for_min_value(name_hash)
(name_hash.max_by {|k, v| 0-v} || [])[0]
end
# or
def key_for_min_value(name_hash)
Array(name_hash.max_by {|k, v| 0-v})[0]
end
I need to iterate over an array and apply a supplied block to each element, and return the first true value returned by the block, which implies that I need to stop immediately as soon as I get a true value.
below is my code. I am a ruby newbie, and I am not sure if this code is reinventing the wheel. Maybe there is a library method or methods that can do that already? or may be this code can be simplified?
RS = {
:x => %w(\d+ a\d+ bb\d+ ccc\d+).map{|x| /^#{x}$/},
:y => %w(\w+ 1\w+ 22\w+ 333\w+).map{|x| /^#{x}$/}
}.freeze
def find s, t
r = RS[s]
if r
r.each do |p|
m = p.match t
return m if m
end
nil
end
end
p find :x, 'bb12345'
If you want the result of the block you could do it this way. This will iterate over the whole array, but wont evaluate any matches after the first one.
def find(s,t)
RS[s].inject(nil) {|m, p| m || p.match(t)}
end
You can break out early doing something like this
RS[s].inject(nil) {|m, p| (m && (break m)) || p.match(t)}
This is duplicated with: Ruby - Array.find, but return the value the block
You want a lazy map:
[nil, 1, 2, 3].lazy.map{|i| i && i.to_s}.find{|i| i}
# => "1"
Hopefully still actual: here a solution using detect, i made it possible to verbose the output so you can see which expressions are evaluated before returning a hit.
def find_match symbol, string , verbose = false, match = nil
if verbose
RS.detect{|x,v|x==symbol;v.detect{|re|puts re;match=string.match(/#{re}/)}}
else
RS.detect{|x,v|x==symbol;v.detect{|re|match=string.match(/#{re}/)}}
end
match
end
p find_match :x, 'bb12345'
p find_match :x, 'ee12345' , true #verbose output
p find_match :x, '12345'
p find_match :y, '22abcd'
#<MatchData "bb12345">
(?-mix:^\d+$)
(?-mix:^a\d+$)
(?-mix:^bb\d+$)
(?-mix:^ccc\d+$)
(?-mix:^\w+$)
#<MatchData "ee12345">
#<MatchData "12345">
#<MatchData "22abcd">
If your regex patterns are simple, you can just apply the regex again at the end, maybe.
Something like:
def find(s,t)
r = RS[s] and r.find{|p| p.match(t)}.try(:match, t)
end
Although it makes one redundant call to match, it is easier to understand.
First, find the pattern you want, then use that pattern.
Here's some example code:
class Obj
attr :c, true
def == that
p '=='
that.c == self.c
end
def <=> that
p '<=>'
that.c <=> self.c
end
def equal? that
p 'equal?'
that.c.equal? self.c
end
def eql? that
p 'eql?'
that.c.eql? self.c
end
end
a = Obj.new
b = Obj.new
a.c = 1
b.c = 1
p [a] | [b]
It prints 2 objects but it should print 1 object. None of the comparison methods get called. How is Array.| comparing for equality?
Array#| is implemented using hashs. So in order for your type to work well with it (as well as with hashmaps and hashsets), you'll have to implement eql? (which you did) and hash (which you did not). The most straight forward way to define hash meaningfully would be to just return c.hash.
Ruby's Array class is implemented in C, and from what I can tell, uses a custom hash table to check for equality when comparing objects in |. If you wanted to modify this behavior, you'd have to write your own version that uses an equality check of your choice.
To see the full implementation of Ruby's Array#|: click here and search for "rb_ary_or(VALUE ary1, VALUE ary2)"
Ruby is calling the hash functions and they are returning different values, because they are still just returning the default object_id. You will need to def hash and return something reflecting your idea of what makes an Obj significant.
>> class Obj2 < Obj
>> def hash; t = super; p ['hash: ', t]; t; end
>> end
=> nil
>> x, y, x.c, y.c = Obj2.new, Obj2.new, 1, 1
=> [#<Obj2:0x100302568 #c=1>, #<Obj2:0x100302540 #c=1>, 1, 1]
>> p [x] | [y]
["hash: ", 2149061300]
["hash: ", 2149061280]
["hash: ", 2149061300]
["hash: ", 2149061280]
[#<Obj2:0x100302568 #c=1>, #<Obj2:0x100302540 #c=1>]
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]