Crystal Notes: Methods

updated: 3rd of August 2021

published: 29th of July 2021

Intro

A Method in Crystal is like a fuction in other languages.

    crystal
  

    # Define a method that accepts no arguments.
def print_stuff
  puts "stuff"
end

# Call a method.
print_stuff()

# Define a method that accepts arguments.
# The parameter types will be inferred by the compiler.
def do_stuff(what, how)
  puts "#{what} #{how}"
end

# Call a method with arguments.
do_stuff("Say hello", "speak")

# Call a method with named parameter arguments.
do_stuff(what: "Say hello", how: "speak")
  

Overloading

Methods in Crystal can be overloaded . This means that methods with the same name but different signatures are treated as different methods.

    crystal
  

    # Define a method and explicity define the types that are accepted.
def do_stuff(what : String, how : String)
  puts "#{what} #{how}"
end

# Call a method with arguments defining the parameters.
do_stuff(what: "Say hello", how: "speak")

# Overload the do_stuff method with a different signature.
def do_stuff(x : Int32, y : Int32)
  puts x + y
end

do_stuff(1, 2)
  

Default Parameters

Methods can have default parameters.

    crystal
  

    # Define a method with default parameters.
def do_stuff(x : Int32, y : Int32 = 1)
  puts x + y
end

# Call a method with default parameters.
do_stuff(1)
  

Splat

A splat parameter is defined by prefixing a parameter with an asterix (*). A splat takes a variadic number of elements.

    crystal
  

    # Define a method with splat parameter.
# The splat items can be optionally type annotated.
def do_stuff(*args : String)
  args.each do |a|
    puts a
  end
end

# Call method with N number of elements.
do_stuff("stuff", "things")

# A Tuple can be splatted into a method
stuff_and_things = {"stuff", "things"}
do_stuff(*stuff_and_things)
  

Double Splat

A double splat parameter is defined by prefixing a parameter with 2 asterixes (**). A double splat takes a variadic number of elements.

    crystal
  

    # Define a double splat method
def do_stuff(**kwargs)
  kwargs.each do |key, value|
    puts "Key: #{key} - Value: #{value}"
  end
end

# Call a method with N number of named arguments.
do_stuff(stuff: "my stuff", things: "my things")

# A NamedTuple can be double splatted into a method.
stuff_and_things = {stuff: "my stuff", things: "my things"}
do_stuff(**stuff_and_things)
  

Return Values

The return type of a method can be specifically defined.

    crystal
  

    # Define a methods return type as a String.
def do_stuff(what : String, how : String) : String
  "#{what} #{how}"
end

# Multiple return values, can be packed in a Tuple or Array.
def do_stuff(what : String, how : String) : Tuple(String, String)
  Tuple(what, how)
end

# Unpack multiple return values
what, how = do_stuff("call glass guy", "on phone")
  

Block

A block lets you reuse code without creating a formal method.

    crystal
  
    # Example of using block.
stuff_and_things.map{ |item| item.upcase }

# (&) is syntastic sugar used to defind the captured object.
stuff_and_things.map(&.upcase)

Proc

A Proc, also know as a lambda or an anonymous function in other languages is a nameless function with a call method. Anonymous functions are defined with the -> operator.

    crystal
  

    # Define an anonymous function and assign it to a 
# variable named fn.
fn = ->(stuff : String, things : String) { "#{stuff} - #{things}" }

# Call anonymous function
fn.call("my stuff", "my things") # => my stuff - my things
  

Considerations

Methods are defined in snake_case by convention.
A splat/double splat can appear only once in any position.
The splat only accepts a tuple.
The double splat only accepts a named tuple.
Methods automatically return the value of the last expression. There is no need to declare the return statement.
Type annotations are required for Procs.
The return type of a Proc is inferred by the compiler.
Crystal does not implement tail-call optimization. Therefore, recursive method calls are not infinitly scalable.