Type restrictions are applied to method parameters to restrict the types accepted by that method.
def add(x : Number, y : Number) x + y end # Ok add 1, 2 # Error: no overload matches 'add' with types Bool, Bool add true, false
Note that if we had defined
add without type restrictions, we would also have gotten a compile time error:
def add(x, y) x + y end add true, false
The above code gives this compile error:
Error in foo.cr:6: instantiating 'add(Bool, Bool)' add true, false ^~~ in foo.cr:2: undefined method '+' for Bool x + y ^
This is because when you invoke
add, it is instantiated with the types of the arguments: every method invocation with a different type combination results in a different method instantiation.
The only difference is that the first error message is a little more clear, but both definitions are safe in that you will get a compile time error anyway. So, in general, it's preferable not to specify type restrictions and almost only use them to define different method overloads. This results in more generic, reusable code. For example, if we define a class that has a
+ method but isn't a
Number, we can use the
add method that doesn't have type restrictions, but we can't use the
add method that has restrictions.
# A class that has a + method but isn't a Number class Six def +(other) 6 + other end end # add method without type restrictions def add(x, y) x + y end # OK add Six.new, 10 # add method with type restrictions def restricted_add(x : Number, y : Number) x + y end # Error: no overload matches 'restricted_add' with types Six, Int32 restricted_add Six.new, 10
Refer to the type grammar for the notation used in type restrictions.
Note that type restrictions do not apply to the variables inside the actual methods.
def handle_path(path : String) path = Path.new(path) # *path* is now of the type Path # Do something with *path* end
Restrictions from instance variables¶
In some cases it is possible to restrict the type of a method's parameter based on its usage. For instance, consider the following example:
class Foo @x : Int64 def initialize(x) @x = x end end
In this case we know that the parameter
x from the initialization function must be an
Int64, and there is no point in leave it unrestricted.
When the compiler finds an assignment from a method parameter to an instance variable, then it inserts such a restriction. In the example above, calling
Foo.new "hi" fails with (note the type restriction):
Error: no overload matches 'Foo.new' with type String Overloads are: - Foo.new(x : ::Int64)
A special type restriction is
class Person def ==(other : self) other.name == name end def ==(other) false end end john = Person.new "John" another_john = Person.new "John" peter = Person.new "Peter" john == another_john # => true john == peter # => false (names differ) john == 1 # => false (because 1 is not a Person)
In the previous example
self is the same as writing
Person. But, in general,
self is the same as writing the type that will finally own that method, which, when modules are involved, becomes more useful.
As a side note, since
Reference the second definition of
== is not needed, since it's already defined in
self always represents a match against an instance type, even in class methods:
class Person getter name : String def initialize(@name) end def self.compare(p1 : self, p2 : self) p1.name == p2.name end end john = Person.new "John" peter = Person.new "Peter" Person.compare(john, peter) # OK
You can use
self.class to restrict to the Person type. The next section talks about the
.class suffix in type restrictions.
Classes as restrictions¶
Using, for example,
Int32 as a type restriction makes the method only accept instances of
def foo(x : Int32) end foo 1 # OK foo "hello" # Error
If you want a method to only accept the type Int32 (not instances of it), you use
def foo(x : Int32.class) end foo Int32 # OK foo String # Error
The above is useful for providing overloads based on types, not instances:
def foo(x : Int32.class) puts "Got Int32" end def foo(x : String.class) puts "Got String" end foo Int32 # prints "Got Int32" foo String # prints "Got String"
Type restrictions in splats¶
You can specify type restrictions in splats:
def foo(*args : Int32) end def foo(*args : String) end foo 1, 2, 3 # OK, invokes first overload foo "a", "b", "c" # OK, invokes second overload foo 1, 2, "hello" # Error foo() # Error
When specifying a type, all elements in a tuple must match that type. Additionally, the empty-tuple doesn't match any of the above cases. If you want to support the empty-tuple case, add another overload:
def foo # This is the empty-tuple case end
A simple way to match against one or more elements of any type is to use
Object as a restriction:
def foo(*args : Object) end foo() # Error foo(1) # OK foo(1, "x") # OK
You can make a type restriction take the type of an argument, or part of the type of an argument, using
def foo(x : T) forall T T end foo(1) # => Int32 foo("hello") # => String
T becomes the type that was effectively used to instantiate the method.
A free variable can be used to extract the type argument of a generic type within a type restriction:
def foo(x : Array(T)) forall T T end foo([1, 2]) # => Int32 foo([1, "a"]) # => (Int32 | String)
To create a method that accepts a type name, rather than an instance of a type, append
.class to a free variable in the type restriction:
def foo(x : T.class) forall T Array(T) end foo(Int32) # => Array(Int32) foo(String) # => Array(String)
Multiple free variables can be specified too, for matching types of multiple arguments:
def push(element : T, array : Array(T)) forall T array << element end push(4, [1, 2, 3]) # OK push("oops", [1, 2, 3]) # Error