Functional Programming For Dynamic Programmers - Part 5
=======================================================

:author: Michele Simionato
:date: December 2007

This is the fifth 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.

Input and Output revisited
--------------------------------------------------------------

In accordance with the example-oriented spirit of this series, I will
introduce functors with a motivating example, again in the arena of
input and output. 
We saw in an earlier installament that the standard library provides two
structures ``TextIO`` and ``BinIO`` for managing text files and binary
files respectively; we also show that the two structures have many things
in common, and it is possibile to define a (sub)signature matching both.


Structures are not first class objects and 
they cannot be passed to and returned from regular functions.
To circumvent this restriction, the authors of ML invented
the concept of *functor*, which is basically a *sui generis* function
taking structures in input and to returning structures in output. Functors
are not-first-class objects themselves and therefore they require a specific
declaration ``functor Name(Struct:SIGNATURE) = funct ... end``.

.. include:: simple-io.aml
   :literal:

The functor here is important, since it shows how it is possible to write
generic code in SML. In this case, I have just written a library which is
able to wrap *any* structure matching the ``SimpleIO`` interface, and I have
avoided a potentially infinite code duplication.

The specification of which specific structure to use will be the job of the client
code, not of the library author; in particular a particular user of the library
may be interested in specializing it both for ``TextIO`` and ``BinIO``
by writing::

 - structure T = ManagedIO(TextIO)
 - structure B = ManagedIO(BinIO)

The operation of specializing a functor is also called *functor instantiation*;
since it happens in a structure declaration it is performed by the compiler
*at compile time*. The advantage is that the compiler can generate different optimized
code for  the structures ``T`` and ``B`` in the *client* program.

 ``- T.withInputFile "three-lines.txt" (print o T.inputAll)``

----

 *Such things are called individuals because each of them consists 
 of characteristics the collection of which will never be the 
 same for anything else. For the characteristics of Socrates will
 never be in any other particular. But the characteristics of man — 
 I mean of the man that is general — will be the same in 
 several things, or rather in all particular men insofar as they 
 are men.*  -- Porphyry