The program is a global object in which you can define types, methods and file-local variables.
# Defines a method in the program def add(x, y) x + y end # Invokes the add method in the program add(1, 2) # => 3
A method's value is the value of its last expression; there's no need for explicit
return expressions. However, explicit
return expressions are possible:
def even?(num) if num % 2 == 0 return true end return false end
When invoking a method without a receiver, like
add(1, 2), it will be searched for in the program if not found in the current type or any of its ancestors.
def add(x, y) x + y end class Foo def bar # invokes the program's add method add(1, 2) # invokes Foo's baz method baz(1, 2) end def baz(x, y) x * y end end
If you want to invoke the program's method, even though the current type defines a method with the same name, prefix the call with
def baz(x, y) x + y end class Foo def bar baz(4, 2) # => 2 ::baz(4, 2) # => 6 end def baz(x, y) x - y end end
Variables declared in a program are not visible inside methods:
x = 1 def add(y) x + y # error: undefined local variable or method 'x' end add(2)
Parentheses in method invocations are optional:
add 1, 2 # same as add(1, 2)
Main code, the code that is run when you compile and run a program, can be written directly in a source file without the need to put it in a special "main" method:
# This is a program that prints "Hello Crystal!" puts "Hello Crystal!"
Main code can also be inside type declarations:
# This is a program that prints "Hello" class Hello # 'self' here is the Hello class puts self end