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Catalog / Crystal Programming Language Cheatsheet

Crystal Programming Language Cheatsheet

A concise reference for the Crystal programming language, covering syntax, data types, control flow, and common standard library features. Designed for quick lookup and efficient development.

Basics & Syntax

Basic Types

Int32

32-bit signed integer (default Int)

Int64

64-bit signed integer

Float32

32-bit floating point number

Float64

64-bit floating point number (default Float)

Bool

true or false

String

UTF-8 encoded character sequence

Char

Unicode code point

Variable Declaration

var name = value - Mutable variable with type inference.
name = new_value - Assigning a new value.

name : Type = value - Mutable variable with explicit type.

CONST_NAME = value - Constant (immutable).

Operators

Arithmetic

+, -, *, /, %, ** (exponentiation)

Comparison

==, !=, >, <, >=, <=

Logical

&&, ||, !

Bitwise

&, |, ^, ~, <<, >>

Assignment

=, +=, -=, *=, /=, %=, **=

Control Flow

Conditional Statements

`if condition

code

end`

`if condition

code

else

code

end`

`if condition

code

elsif condition

code

else

code

end`

`unless condition

code

end(opposite ofif`)

Looping

`while condition

code

end`

`until condition

code

end(opposite ofwhile`)

`loop do

code

break if condition
end`

(0..5).each do |i| puts i end

Case Statement

`case value
when condition1

code

when condition2

code

else

code

end`

Methods & Blocks

Method Definition

`def method_name(arg1 : Type, arg2 : Type)

code

return value
end`

`def method_name(arg1 : Type, arg2 : Type) : ReturnType

code

return value
end`

def method_name = value (shorthand for simple methods)

Methods can have default argument values: def method_name(arg1 : Type = default_value)

Blocks & Procs

Blocks are anonymous functions passed to methods.
`method do |arg1, arg2|

code

end`

Procs are first-class blocks.
`my_proc = proc do |arg1, arg2|

code

end my_proc.call(value1, value2)`

Lambdas are similar to procs but enforce argument arity.
`my_lambda = ->(arg1, arg2) {

code

} my_lambda.(value1, value2)`

Classes & Modules

Class Definition

`class ClassName

code

end`

`class ClassName < SuperClass

code

end` (inheritance)

initialize method is the constructor.

Instance variables: @variable

Module Definition

`module ModuleName

code

end`

Modules can be used for namespacing and mixins.

include ModuleName - includes module methods as instance methods.

extend ModuleName - includes module methods as class methods.

Structs

struct Point property x : Int32 property y : Int32 end

Structs are value types (passed by value).
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