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June 20, 2019 13:22
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List comprehension in ruby
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require 'pry-byebug' | |
module ListComprehension | |
def self.execute(block) | |
context = OuterBlockContextClass.new | |
# Set up variables + blocks | |
context.instance_eval(&block) | |
# evaluate the comprehension | |
context.run | |
end | |
# Collects variable names and sets | |
class Variable | |
attr_reader :name, :set | |
def initialize(name) | |
@name = name | |
end | |
def <<(set) | |
@set = set | |
self | |
end | |
end | |
# Wrapper class to make the pipe syntax work | |
class PipeTo | |
def initialize(action) | |
@action = action | |
end | |
def |(*args) | |
@action[*args] | |
end | |
end | |
# The context in which the comprehension will be evaluated. | |
class OuterBlockContextClass | |
# Make variable declarations like `x` and `k` work. | |
def method_missing(symbol, *args) | |
Variable.new(symbol) | |
end | |
# Declare one of the two proc sections (either condition or map) | |
def p(&block) | |
if @map | |
# must be last section | |
@condition = block | |
else | |
# Must be first section | |
@map = block | |
PipeTo.new(proc { |variables| @variables = variables; PipeTo.new(proc{}) }) | |
end | |
end | |
# Makes a proc that evaluates `prok` in the context of our @variables | |
def make_scoped_proc(prok) | |
proc do |args| | |
o = Object.new | |
@variables.zip(args).each do |variable, value| | |
o.define_singleton_method(variable.name) { value } | |
end | |
o.instance_eval(&prok) | |
end | |
end | |
# Run the comprehension, after everything's all set up. | |
def run | |
comp( | |
sets: @variables.map(&:set).map(&:to_a), | |
condition: make_scoped_proc(@condition), | |
map: make_scoped_proc(@map) | |
) | |
end | |
# *Atually* run the comprehension. | |
def comp(sets: [], condition: proc{}, map: proc{}) | |
sets[0] | |
.product(*sets[1..]) | |
.select(&condition) | |
.map(&map) | |
end | |
end | |
end | |
# Shorthand for comprehensions. Todo: maybe find a good operator to overload | |
# for this instead? Would love to overload the array literal `[]` syntax if I | |
# can figure out how. | |
def c(&block) | |
ListComprehension.execute(block) | |
end | |
puts c{ p{ x + k } | [ x << (1...5), k << (1...10) ] | p{ k % x == 0 } } |
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Just a fun experiment in metaprogramming and syntax hacking. This is about as succinct as I could get a list comprehension syntax in ruby. Open to suggestions for improvements!