path: root/ml/objects.txt

Functional Programming For Dynamic Programmers - Part 6
=======================================================================

:author:	Michele Simionato
:date:	December 2007

This is the sixth of a series of articles on functional programming in
statically typed languages. It is intended for dynamic programmers,
i.e. for programmers with a background in dynamically typed languages,
such as Perl, Python, Ruby, or languages in the Lisp family. The
approch is eminently practical and example-based; the main goal is to
see if we can stole some good idea from statically typed languages. In
order to be concrete, I will consider languages in the ML family,
because they are pretty nice and much easier to understand that
Haskell.

Records
-----------------------

The first level of grouping in ML is the record level: a record
roughly correspond to a *struct* in the C programming language and can
contain any kind of first class value. The record itself a first class value,
i.e. it can be passed to and returned from functions. I will give a few
examples of usage.

A record factory::

 - fun makeArticle {title:string, author:string} = {title, author};
 val makeArticle :  {author : string, title : string} -> {author : string, title : string} = _fn

An example record::

 - val article = makeArticle {title="Functional Programming", author="M. Simionato"};
 val article : {author : string, title : string} =
   {author = "M. Simionato", title = "Functional Programming"}

Extracting fields from a record::

 - #title article;
 val it : string = "Functional Programming"
 - #author article;
 val it : string = "M. Simionato"

A record containing functions::

 - val a = {makeArticle, printArticle = fn {title, author} => print (title^" "^author ^"\n") };
 - #printArticle a article;
 Functional Programming M. Simionato

Notice that the order of the fields is not specified and that there is
no concept of subrecord; for instance two records of kind
``{title:string, author:string, publicationDate:string}`` and
``{title:string, author:string}`` are considered completely different
record types, the second one is not a subtype of the first one and you
cannot substitute one with the other, possibly with default
values. Also, records are purely static and resolved at compile time:
if you need something more dynamic, you should use a map, not a
record. Finally, records are functional, i.e. immutable: there is no
way to change the value of a field, you must create an entirely new
record with a different value if you want to simulate a record update.
Alice provides some syntactic sugar for functional record update;
here is an example::

 - val r={a=1,b=2};
 val r : {a : int, b : int} = {a = 1, b = 2}
 - {r where a=2};
 val it : {a : int, b : int} = {a = 2, b = 2}


fun enum n = lazy n :: enum (n+1)

(for instance, a log file); how can we process it? The simplest possibility is
to convert it into a lazy list of lines, with the following code::

 fun textFileToList filename = let
     val file = TextIO.openIn filename
     fun lazy readLines () =
         case TextIO.inputLine file 
               handle ex => (TextIO.closeIn file; raise ex)
          of NONE => (TextIO.closeIn file; [])
           | SOME line => line :: readLines ()
 in
     readLines ()
 end


A smarter line counter
-----------------------------------------------------------

 - val countLines = length o textFileToList 




Python extended generators in Alice
---------------------------------------------------------

def consumer():
  result = []
   while True:
     inp = yield
     if inp = END:
       return result 

functor consumer(val ch: channel):
   while true do let
      val inp = Channel.gexst ch
      if inp = END:


In particular, what's the equivalent of the simple
Python one-liner ``for item in container: print item``?  The answer is
that there is no equivalent. The Python one-liner is completely
polymorphic, since it works for any kind of container and it is able
to print any kind of object. However, SML is a statically typed
language, and you must give to the compiler some information about the
types: it cannot predict the future, nor the type of the container and
the types of the items.  Here I will show how you can loop on
homogenous containers, i.e . containers where the items are all
instances of the same type;


 fun writeln (tostring, value)  =
    print(format (tostring o text "\n") value);


-  app (fn item => writeln(int, item)) [1,2,3];
1
2
3

fun sum lst = foldl op+ 0 lst

The thing to remember is that functions associate to the **right**, so

  ``a -> b -> c``

means

  ``a -> (b -> c)``

whereas type constructor associates to the **left**, so 

  ``int ref list``

means

   ``(int ref) list``


Timers
--------------------

fun timedFn f a = let
  val ct = Timer.startRealTimer ()
  val result = f a
in
  (result, Timer.checkRealTimer (ct))
end


 - structure A = struct
     type 'a aggr 
     fun length(a: 'a aggr) = 1
     fun sub(a: 'a aggr, i :int) = 1
   end;
 signature AGGREGATE_TYPE =
   sig
      type 'a aggr 
      val length : 'a aggr -> int
      val sub : 'a aggr * int -> int
   end


Higher order functions can also be used to implement poor man's object
systems; consider for instance this example::

 class Counter(object):
    def __init__(self, initial_value):
         self.initial_value = self.value = 0
    def incr(self):
         self.value += 1
    def show(self):
         print "The value is %d" % self.value 
    def reset(self):
         self.value = self.initial_value

We can get the same effect via a stateful higher order function
(a.k.a. a closure)::

 exception MissingMethod

 fun makeCounter initialValue = let
    val value = ref initialValue
 in 
     fn "reset" => value := initialValue
      | "show" => (print The value is "; print (Int.toString (!value)); print "\n")
      | "incr" => value := !value + 1
      | _ => raise MissingMethod
 end

 val counter = makeCounter 0;

 do counter "incr";
 do counter "show"; (* The value is 1 *)
 do counter "reset";
 do counter "show"; (* The value is 0 *)

This example also shows the usage of pattern matching and of exceptions.

You can rewrite it in a functional way as follows:

----

*The venerable master Qc Na was walking with his student, Anton.  Hoping to
prompt the master into a discussion, Anton said "Master, I have heard that
objects are a very good thing - is this true?"  Qc Na looked pityingly at
his student and replied, "Foolish pupil - objects are merely a poor man's
closures."*

*Chastised, Anton took his leave from his master and returned to his cell,
intent on studying closures.  He carefully read the entire "Lambda: The
Ultimate..." series of papers and its cousins, and implemented a small
Scheme interpreter with a closure-based object system.  He learned much, and
looked forward to informing his master of his progress.*

*On his next walk with Qc Na, Anton attempted to impress his master by
saying "Master, I have diligently studied the matter, and now understand
that objects are truly a poor man's closures."  Qc Na responded by hitting
Anton with his stick, saying "When will you learn? Closures are a poor man's
object."  At that moment, Anton became enlightened.*

      -- Anton van Straaten