Numbered Block Parameters: 1 Is the New $

2026-03-23

Ruby 2.7 introduced _1, _2, _3 as implicitly numbered block parameters. Skip naming your block variables entirely. No Perl equivalent exists for this one. It's Ruby going full minimalist, and it's glorious for short blocks where a variable name adds nothing.

Part 1: Basic Usage

# Instead of naming the block variable:
lines.map { |line| line.strip }

# Use _1:
lines.map { _1.strip }

# Instead of two named variables:
hash.select { |key, value| value > 10 }

# Use _1 and _2:
hash.select { _2 > 10 }

Part 2: When to Use Them

Best for short, simple blocks where naming the variable adds no clarity:

# Good - simple operations
lines.map { _1.chomp }
pairs.map { _1 + _2 }
hash.select { _2 > 10 }
lines.map { _1.split[2] }       # grab third field from each line

# Less good - complex operations where names help readability
# Use named params when the logic is complex:
records.map { |record| record.split(",")[3].strip.to_i * 100 }

Part 3: With Various Methods

# map
nums.map { _1 * 2 }

# select/reject
words.select { _1.length > 5 }
lines.reject { _1.empty? }

# each
lines.each { puts _1 }

# sort_by
words.sort_by { _1.length }

# With two parameters
hash.each { puts "#{_1}: #{_2}" }
pairs.map { "#{_1}=#{_2}" }

Part 4: Combining with Other Features

# With regex
lines.select { _1 =~ %r~ERROR~ }

# With method chaining
File.readlines("log.txt")
  .map { _1.chomp }
  .select { _1 =~ %r~error~i }
  .map { _1.split[0] }          # grab timestamp

# With then/yield_self
data.then { JSON.parse(_1) }.then { _1["users"] }

Part 5: The Rules

You can't mix numbered params with named params in the same block:

# ERROR:
hash.map { |key| "#{key}: #{_2}" }  # can't mix!

_1 through _9 are available, but if you need more than 2-3, use named params for clarity. If you're using _4, you should probably rethink your approach.

Part 6: Symbol-to-Proc vs Numbered Params

# Symbol-to-proc: for no-argument method calls
lines.map(&:chomp)           # cleanest for simple methods

# Numbered params: for anything more complex
lines.map { _1.split(",")[2] }  # can't do this with &:
lines.select { _1.length > 80 } # needs an argument

Rule of thumb: use &:method when calling a single no-argument method. Use _1 when you need to do something more with the value. Use named params when the logic is complex enough that names help readability.