Crystal Cheat Sheet

Core Crystal syntax covering static typing, classes, collections, and blocks for a Ruby-like compiled language.

2 PagesIntermediateApr 12, 2026

Basics

Variables, interpolation, and methods.

crystal

# Variables and printingname = "World"puts "Hello, #{name}!"      # string interpolationx = 42                        # type inferred as Int32def add(a, b)  a + bendputs add(2, 3)

Static Types

Explicit types, nilable types, and unions.

crystal

x : Int32 = 42y : Float64 = 3.14s : String = "text"n : Int32? = nil             # nilable type (union with Nil)def describe(x : Int32 | String)  case x  when Int32    puts "int: #{x}"  when String    puts "string: #{x}"  endend

Classes

Inheritance and instance variables.

crystal

class Animal  property name : String  def initialize(@name : String)  end  def speak    "#{name} makes a sound"  endendclass Dog < Animal  def speak    "#{name} barks"  endendputs Dog.new("Rex").speak

Collections

Arrays and hashes.

[1, 2, 3]- Array(Int32) literal
{"a" => 1, "b" => 2}- Hash(String, Int32) literal
arr.map { |x| x * 2 }- Transforms each element
arr.select { |x| x > 1 }- Filters elements matching a condition
arr.each { |x| puts x }- Iterates over elements
arr << 4- Appends an element (alias for push)

Blocks & Procs

Yielding and typed procs.

def each(&block)- Method that accepts a block argument
yield x- Invokes the block passed to the current method
->(x : Int32) { x * 2 }- Typed Proc literal
arr.each_with_index { |x, i| }- Iterates with an index
1.upto(5) { |i| }- Iterates an inclusive numeric range
String.build { |s| s << "x" }- Efficiently builds a string

Pro Tip

Crystal's type inference is compile-time and static despite Ruby-like syntax — build with crystal build --release for optimized binaries, since debug builds are noticeably slower.

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