| language | filename | contributors | ||||||
|---|---|---|---|---|---|---|---|---|
crystal |
learncrystal.cr |
|
# This is a comment
# Everything is an object
nil.class #=> Nil
100.class #=> Int32
true.class #=> Bool
# Falsey values are: nil, false and null pointers
!nil #=> true : Bool
!false #=> true : Bool
!0 #=> false : Bool
# Integers
1.class #=> Int32
# Four signed integer types
1_i8.class #=> Int8
1_i16.class #=> Int16
1_i32.class #=> Int32
1_i64.class #=> Int64
# Four unsigned integer types
1_u8.class #=> UInt8
1_u16.class #=> UInt16
1_u32.class #=> UInt32
1_u64.class #=> UInt64
2147483648.class #=> Int64
9223372036854775808.class #=> UInt64
# Binary numbers
0b1101 #=> 13 : Int32
# Octal numbers
0o123 #=> 83 : Int32
# Hexadecimal numbers
0xFE012D #=> 16646445 : Int32
0xfe012d #=> 16646445 : Int32
# Floats
1.0.class #=> Float64
# There are two floating point types
1.0_f32.class #=> Float32
1_f32.class #=> Float32
1e10.class #=> Float64
1.5e10.class #=> Float64
1.5e-7.class #=> Float64
# Chars
'a'.class #=> Char
# Octal codepoint
'\101' #=> 'A' : Char
# Unicode codepoint
'\u0041' #=> 'A' : Char
# Strings
"s".class #=> String
# Strings are immutable
s = "hello, " #=> "hello, " : String
s.object_id #=> 134667712 : UInt64
s += "Crystal" #=> "hello, Crystal" : String
s.object_id #=> 142528472 : UInt64
# Supports interpolation
"sum = #{1 + 2}" #=> "sum = 3" : String
# Multiline string
"This is
multiline string"
# String with double quotes
%(hello "world") #=> "hello \"world\""
# Symbols
# Immutable, reusable constants represented internally as Int32 integer value.
# They're often used instead of strings to efficiently convey specific,
# meaningful values
:symbol.class #=> Symbol
sentence = :question? # :"question?" : Symbol
sentence == :question? #=> true : Bool
sentence == :exclamation! #=> false : Bool
sentence == "question?" #=> false : Bool
# Arrays
[1, 2, 3].class #=> Array(Int32)
[1, "hello", 'x'].class #=> Array(Int32 | String | Char)
# Empty arrays should specify a type
[] # Syntax error: for empty arrays use '[] of ElementType'
[] of Int32 #=> [] : Array(Int32)
Array(Int32).new #=> [] : Array(Int32)
# Arrays can be indexed
array = [1, 2, 3, 4, 5] #=> [1, 2, 3, 4, 5] : Array(Int32)
array[0] #=> 1 : Int32
array[10] # raises IndexError
array[-6] # raises IndexError
array[10]? #=> nil : (Int32 | Nil)
array[-6]? #=> nil : (Int32 | Nil)
# From the end
array[-1] #=> 5
# With a start index and size
array[2, 3] #=> [3, 4, 5]
# Or with range
array[1..3] #=> [2, 3, 4]
# Add to an array
array << 6 #=> [1, 2, 3, 4, 5, 6]
# Remove from the end of the array
array.pop #=> 6
array #=> [1, 2, 3, 4, 5]
# Remove from the beginning of the array
array.shift #=> 1
array #=> [2, 3, 4, 5]
# Check if an item exists in an array
array.includes? 3 #=> true
# Special syntax for an array of string and an array of symbols
%w(one two three) #=> ["one", "two", "three"] : Array(String)
%i(one two three) #=> [:one, :two, :three] : Array(Symbol)
# There is a special array syntax with other types too, as long as
# they define a .new and a #<< method
set = Set{1, 2, 3} #=> [1, 2, 3]
set.class #=> Set(Int32)
# The above is equivalent to
set = Set(typeof(1, 2, 3)).new
set << 1
set << 2
set << 3
# Hashes
{1 => 2, 3 => 4}.class #=> Hash(Int32, Int32)
{1 => 2, 'a' => 3}.class #=> Hash(Int32 | Char, Int32)
# Empty hashes should specify a type
{} # Syntax error
{} of Int32 => Int32 # {}
Hash(Int32, Int32).new # {}
# Hashes can be quickly looked up by key
hash = {"color" => "green", "number" => 5}
hash["color"] #=> "green"
hash["no_such_key"] #=> Missing hash key: "no_such_key" (KeyError)
hash["no_such_key"]? #=> nil
# Check existence of keys hash
hash.has_key? "color" #=> true
# Special notation for symbol and string keys
{key1: 'a', key2: 'b'} # {:key1 => 'a', :key2 => 'b'}
{"key1": 'a', "key2": 'b'} # {"key1" => 'a', "key2" => 'b'}
# Special hash literal syntax with other types too, as long as
# they define a .new and a #[]= methods
class MyType
def []=(key, value)
puts "do stuff"
end
end
MyType{"foo" => "bar"}
# The above is equivalent to
tmp = MyType.new
tmp["foo"] = "bar"
tmp
# Ranges
1..10 #=> Range(Int32, Int32)
Range.new(1, 10).class #=> Range(Int32, Int32)
# Can be inclusive or exclusive
(3..5).to_a #=> [3, 4, 5]
(3...5).to_a #=> [3, 4]
# Check whether range includes the given value or not
(1..8).includes? 2 #=> true
# Tuples are a fixed-size, immutable, stack-allocated sequence of values of
# possibly different types.
{1, "hello", 'x'}.class #=> Tuple(Int32, String, Char)
# Acces tuple's value by its index
tuple = {:key1, :key2}
tuple[1] #=> :key2
tuple[2] #=> syntax error : Index out of bound
# Can be expanded into multiple variables
a, b, c = {:a, 'b', "c"}
a #=> :a
b #=> 'b'
c #=> "c"
# Procs represent a function pointer with an optional context (the closure data)
# It is typically created with a proc litteral
proc = ->(x : Int32) { x.to_s }
proc.class # Proc(Int32, String)
# Or using the new method
Proc(Int32, String).new { |x| x.to_s }
# Invoke proc with call method
proc.call 10 #=> "10"
# Control statements
if true
"if statement"
elsif false
"else-if, optional"
else
"else, also optional"
end
puts "if as a suffix" if true
# If as an expression
a = if 2 > 1
3
else
4
end
a #=> 3
# Ternary if
a = 1 > 2 ? 3 : 4 #=> 4
# Case statement
cmd = "move"
action = case cmd
when "create"
"Creating..."
when "copy"
"Copying..."
when "move"
"Moving..."
when "delete"
"Deleting..."
end
action #=> "Moving..."
# Loops
index = 0
while index <= 3
puts "Index: #{index}"
index += 1
end
# Index: 0
# Index: 1
# Index: 2
# Index: 3
index = 0
until index > 3
puts "Index: #{index}"
index += 1
end
# Index: 0
# Index: 1
# Index: 2
# Index: 3
# But the preferable way is to use each
(1..3).each do |index|
puts "Index: #{index}"
end
# Index: 0
# Index: 1
# Index: 2
# Index: 3
# Variable's type depends on the type of the expression
# in control statements
if a < 3
a = "hello"
else
a = true
end
typeof a #=> (Bool | String)
if a && b
# here both a and b are guaranteed not to be Nil
end
if a.is_a? String
a.class #=> String
end
# Functions
def double(x)
x * 2
end
# Functions (and all blocks) implicitly return the value of the last statement
double(2) #=> 4
# Parentheses are optional where the call is unambiguous
double 3 #=> 6
double double 3 #=> 12
def sum(x, y)
x + y
end
# Method arguments are separated by a comma
sum 3, 4 #=> 7
sum sum(3, 4), 5 #=> 12
# yield
# All methods have an implicit, optional block parameter
# it can be called with the 'yield' keyword
def surround
puts '{'
yield
puts '}'
end
surround { puts "hello world" }
# {
# hello world
# }
# You can pass a block to a function
# "&" marks a reference to a passed block
def guests(&block)
block.call "some_argument"
end
# You can pass a list of arguments, which will be converted into an array
# That's what splat operator ("*") is for
def guests(*array)
array.each { |guest| puts guest }
end
# If a method returns an array, you can use destructuring assignment
def foods
["pancake", "sandwich", "quesadilla"]
end
breakfast, lunch, dinner = foods
breakfast #=> "pancake"
dinner #=> "quesadilla"
# By convention, all methods that return booleans end with a question mark
5.even? # false
5.odd? # true
# And if a method ends with an exclamation mark, it does something destructive
# like mutate the receiver. Some methods have a ! version to make a change, and
# a non-! version to just return a new changed version
company_name = "Dunder Mifflin"
company_name.gsub "Dunder", "Donald" #=> "Donald Mifflin"
company_name #=> "Dunder Mifflin"
company_name.gsub! "Dunder", "Donald"
company_name #=> "Donald Mifflin"
# Define a class with the class keyword
class Human
# A class variable. It is shared by all instances of this class.
@@species = "H. sapiens"
# type of name is String
@name : String
# Basic initializer
# Assign the argument to the "name" instance variable for the instance
# If no age given, we will fall back to the default in the arguments list.
def initialize(@name, @age = 0)
end
# Basic setter method
def name=(name)
@name = name
end
# Basic getter method
def name
@name
end
# The above functionality can be encapsulated using the attr_accessor method as follows
property :name
# Getter/setter methods can also be created individually like this
getter :name
setter :name
# A class method uses self to distinguish from instance methods.
# It can only be called on the class, not an instance.
def self.say(msg)
puts msg
end
def species
@@species
end
end
# Instantiate a class
jim = Human.new("Jim Halpert")
dwight = Human.new("Dwight K. Schrute")
# Let's call a couple of methods
jim.species #=> "H. sapiens"
jim.name #=> "Jim Halpert"
jim.name = "Jim Halpert II" #=> "Jim Halpert II"
jim.name #=> "Jim Halpert II"
dwight.species #=> "H. sapiens"
dwight.name #=> "Dwight K. Schrute"
# Call the class method
Human.say("Hi") #=> print Hi and returns nil
# Variables that start with @ have instance scope
class TestClass
@var = "I'm an instance var"
end
# Variables that start with @@ have class scope
class TestClass
@@var = "I'm a class var"
end
# Variables that start with a capital letter are constants
Var = "I'm a constant"
Var = "can't be updated" # Already initialized constant Var
# Class is also an object in crystal. So class can have instance variables.
# Class variable is shared among the class and all of its descendants.
# base class
class Human
@@foo = 0
def self.foo
@@foo
end
def self.foo=(value)
@@foo = value
end
end
# derived class
class Worker < Human
end
Human.foo #=> 0
Worker.foo #=> 0
Human.foo = 2 #=> 2
Worker.foo #=> 0
Worker.foo = 3 #=> 3
Human.foo #=> 2
Worker.foo #=> 3
module ModuleExample
def foo
"foo"
end
end
# Including modules binds their methods to the class instances
# Extending modules binds their methods to the class itself
class Person
include ModuleExample
end
class Book
extend ModuleExample
end
Person.foo # => undefined method 'foo' for Person:Class
Person.new.foo # => 'foo'
Book.foo # => 'foo'
Book.new.foo # => undefined method 'foo' for Book
# Exception handling
# Define new exception
class MyException < Exception
end
# Define another exception
class MyAnotherException < Exception; end
ex = begin
raise MyException.new
rescue ex1 : IndexError
"ex1"
rescue ex2 : MyException | MyAnotherException
"ex2"
rescue ex3 : Exception
"ex3"
rescue ex4 # catch any kind of exception
"ex4"
end
ex #=> "ex2"