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script:walkthrough:functions [2017-03-20 22:26]
skyjake
script:walkthrough:functions [2019-11-25 08:41] (current)
skyjake [Defining into a record]
Line 1: Line 1:
 +<- Records ^ ^ Scopes ->
 +
 +====== Functions ======
 +
 +A function is defined in the current scope using a ''​def''​ statement. ''​def''​ is a compound statement, so it must be either a single-line compound or ''​end''​ must be used in the end.
 +
 +  def func1(): pass
 +  ​
 +  def func2()
 +  end
 +
 +The ''​return''​ statement is used to return a value from a function.
 +
 +  def func3(): return '​World'​
 +  print '​Hello',​ func3()
 +
 +
 +===== Arguments =====
 +
 +Functions can be defined with a list of untyped arguments.
 +
 +  def func5(a, b, c)
 +
 +Default values can be included. ​
 +
 +  def func5(a, b='​default',​ c=100)
 +
 +When calling a function, all arguments must have values. Argument names can always be included in the function call, regardless of whether they were defined with default values or not. When using names, the order can also be changed.
 +
 +  func5(c=5, a=100, b='​OOO'​)
 +
 +
 +===== Records as arguments/​return value =====
 +
 +Records are passed by reference both in arguments and in the return value. This means that if a record is created as a local variable, it will be deleted even though it is returned.
 +
 +  def func4()
 +      record a          # '​a'​ owns the record
 +      a.value = "​func4"​
 +      return a          # non-owned reference returned
 +  end
 +  print func4().value ​  # throws NullError!
 +
 +In this case, one could instead use the ''​Record()''​ function to return a copy of the record to the caller.
 +
 +  return Record(a)
 +
 +When passing a record as argument, the function gets a non-owned reference.
 +
 +  record myrec
 +  myrec.val = '​Original'​
 +  print myrec.val
 +  def modify(r)
 +      r.val = '​Changed'​
 +  end
 +  modify(myrec)
 +  print myrec.val
 +
 +Output:
 +
 +  Original
 +  Changed
 +
 +Note that records are always passed by reference, even when specifically passing in an owned reference as argument. In the example below, the created record exists only for the duration of the function call. The function'​s local variable //rec// is initialized with a non-owned reference to the record. This non-owned reference is returned, and thus an invalid reference will be assigned to //a//. Trying to access //a// after this would return in a ''​NullError''​ exception.
 +
 +  def do_init(rec)
 +      rec.val = '​inited'​
 +      return rec
 +  end
 +  # The following should be avoided:
 +  a = do_init(Record()) ​ # returns null ref!
 +
 +<​note>​It is advisable to not use ''​Record()''​ in arguments. Only use it to assign an owned reference to a local variable, or when returning a copy of a record.</​note>​
 +
 +===== Defining into a record =====
 +
 +The ''​def''​ statement accepts identifier lookups within the function name. For example, one could define the function //func5// as a member of //myrec//.
 +
 +  def myrec.func5(a,​ b)
 +      return a + b
 +  end
 +
 +In fact, ''​def''​ behaves like an assignment operator. The above code means:
 +  - Look up //myrec//.
 +  - Create a variable called //func5// within //myrec//.
 +  - Create a function object taking arguments (a, b) and assign it as the value of the variable //​myrec.func5//​.
 +
 +Since functions are actually variables that reference function objects, function definitions can be made within the scope of a function.
 +
 +  def func6()
 +      def func7(): return 'local function'​
 +  end
 +
 +//func7// is not visible outside //func6//, just like any other local variable. However, one could return //func7// out of //func6// and then it could be used just like a regular top-level function.
 +
 +Functions defined inside another function do not have visibility to variables in the outer function. Instead, each function has its own local namespace and may access the module'​s global namespace.
 +
 +  a = '​global scope'
 +  def outer()
 +      a = 100
 +      print '​outer:​ a =', a
 +      def inner()
 +          print '​inner:​ a =', a
 +      end
 +      inner()
 +  end
 +  outer()
 +
 +Output:
 +
 +  outer: a = 100
 +  inner: a = global scope
 +
 +
 +===== Function variables =====
 +
 +Since functions are just variables referencing function objects, one can trivially make an alias for a function with assignment. Unlike records, function objects are reference-counted,​ so a function remains available as long as there is at least one variable pointing to it.
 +
 +  $ def someFunc(): return '​works'​
 +  ​
 +  $ other = someFunc
 +  ​
 +  $ print someFunc(), other()
 +  works works
 +
 +Functions can be passed as arguments.
 +
 +  def doCallback(a,​ b, cb)
 +      return cb(a, b)
 +  end
 +  doCallback(10,​ 15, workFunction)