A gentle introduction to reflection

Lecture 01 : 
A Gentle Introduction to
Reflection
LSINF 2335
Programming Paradigms
Prof. Kim Mens
UCL / EPL / INGI
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Motivation
LSINF 2335
Prof. Kim Mens

LSINF2335:Prog.Paradigms:
Theory,Pract.andApplic. Programming Paradigms:
Theory, Practice and Applications
■ Safir-Whorf Hypothesis (linguistics)
– A particular language's nature influences the habitual
thought of its speakers: different language patterns
yield different patterns of thought.
■ Consequence:
– language influences the thoughts of people who use it
– language determines how we see and think about the world
– the limits of one's language become the limits of one's world
3

■ Safir-Whorf Hypothesis applied to programming
– The programming language used determines how we see and
think about the solution.
■ Is a good programmer as efficient regardless of the
language used?
– Probably not; neither is the program...
■ A programming language is a problem-solving tool
– Choose right programming language to solve your problem
– A good language still doesn’t make a good programmer...
• ... but it can make a bad programmer perform well
• And make a good programmer perform better
4

5
“When you only have a hammer, everything looks like a nail”
■ Do you only know 1 or 2 languages / paradigms?
– Is that really the situation you want to be in when
developing ?
– Aren’t we supposed to be the computer science experts?
■ This course focusses on advanced programming
languages, paradigms and techniques
– Selected theme: reflection, metaprogramming 
and a bit of context-oriented programming.

Reflection
LSINF 2335
Prof. Kim Mens

Theory,Pract.andApplic.
Reflection (language)
■ Meaning
– one of the meanings of reflection (in English) is
“introspection” : contemplation of one-self
– in other words, reflection is the ability of a thing to talk about
itself
■ Reflection in natural language
– “This sentence contains 37 characters.”
– “This sentence contains 27 letters.”
■ Reflection can easily introduce paradoxes
– “This sentence is false.”
• Suppose true. Then claim is correct. So false. Contradiction.
• Suppose false. Then claim is wrong. So true. Contradiction.
7

Reflection (art)
8

9

Reflection (computing)
■ In computer science, (computational) reflection is the
ability of a program to examine and control its own
implementation
■ Reflective programming
– is the programming paradigm driven by reflection
– is a special case of metaprogramming*
■ Computational reflection is good for (a.o.)
– extending a language / language design
– building programming environments
– advanced software development tools
– building knowledge and learning systems
– self-modifying / self-optimising applications
10
* see later

■ Reflection is a pretty advanced feature
– requires strong grasp of the fundamentals of the language
– allows to solve problems elegantly, that were
• previously handled on an ad-hoc basis
• or hard / impossible to achieve
■ Be careful not to introduce paradoxes
– It is possible to write programs that modify themselves while
being executed
– If you’re not careful this can lead to weird situations
– Here’s an example you all know:
• Modifying a data-structure while iterating over it.
• And that’s not even reflection...
11

Reflective languages
■ Many programmers (and some programming languages) do not
fully recognise the importance of reflection
– for example, debugging a program by manually adding
debugging statements all over
■ Some languages provide some types of reflection
– for debugging or error handling only
– or only introspection but no real modification of existing
programs
■ Aspect-oriented languages take it a step further
– but still limited by the set of constructs they offer
12

■ Fully reflective languages offer the full power of
reflection
– allow to reason about and modify nearly any aspect of a running
program
■ Today, many programming languages offer interesting
reflective capabilities
– common in high-level virtual machine programming languages like
Smalltalk
• note: Java is not fully reflective (mainly introspection)
– less common in lower-level programming languages like C
13

■ Have you ever thought of...
– writing a program to generate a visitor class, given a class
hierarchy ?
– adding methods on a class while your code is running ?
– finding out all the instances of a particular class at runtime, or
– all messages understood by an object ?
– adding new keywords to your language ?
■ In a fully reflective language like Smalltalk you can do
all that...
■ ... and more.
14

Metaprogramming
LSINF 2335
Prof. Kim Mens

■ In computer science, (computational) reflection is the
ability of a program to examine and control its own
implementation
■ Reflective programming
– is the programming paradigm driven by reflection
– is a special case of metaprogramming
■ Computational reflection is good for (a.o.)
– extending a language / language design
– building programming environments
– advanced software development tools
– building knowledge and learning systems
– self-modifying / self-optimising applications
16

Meta (language)
■ In Greek
– The preposition µετά means "after", "beside" or "with"
■ In English
– the prefix meta- means “about” (its own category)
– for example, metadata are data about data
• (who has produced it, when, what format are the data in, ...)
– Douglas Hofstadter popularised the meaning of the term
• For example, “going meta”
– rethorical trick to take a debate or analysis to a higher
abstraction level
– “This discussion isn’t really going anywhere.”
• His books “Gödel, Escher, Bach” and “Metamagical Themas” are
a must-read (for any self-reflecting computer scientist)
17

Meta (computing)
■ In computer science
– metaprogramming is the writing of computer programs that
write or manipulate other programs as their data
– reflective programming is a case of metaprogramming
• where a program is its own metaprogram
• i.e., when a program manipulates itself
■ Well-known examples of metaprogramming
– a compiler
– an interpreter
– a code analysis tool
– a code generator
– ... 18

Quine (the philosopher)
■ Willard Van Orman Quine (1908–2000)
■ Philosopher who made an extensive study of indirect
self-reference
■ Quine’s paradox
– ‘Yields falsehood when preceded by its quotation’
– Can you see the paradox?
19

Quine (the philosopher)
■ Willard Van Orman Quine (1908–2000)
■ Philosopher who made an extensive study of indirect
self-reference
■ Quine’s paradox
– ‘Yields falsehood when preceded by its quotation’
– “ ‘Yields falsehood when preceded by its quotation’ yields
falsehood when preceded by its quotation ”
• Suppose true. Then sentence states falsehood. Contradiction.
• Suppose false. Then sentence is wrong. So it does yield falsehood.
Therefore sentence must be true. Contradiction.
20

Quine (programs)
■ In computing a quine is a program that produces its
complete source code as its only output
■ A quine is a metaprogram
– can be a reflective program
■ Some restrictions
– program cannot take input
– empty programs are not allowed
• The empty quine once won the “worst abuse of the rules” price in the
Obfuscated C contest
■ The Quine page
– http://www.nyx.net/~gthompso/quine.htm
– http://en.wikipedia.org/wiki/Quine_(computing)
21

Quine (Javascript)
■ Here’s a quine in Javascript
function a() { document.write (a + "na();"); }
a();
■ almost trivial because Javascript gives you access to a
function’s source code
22

Bootstrapping
■ An allusion to lift yourself up by your own bootstraps
■ Bootstrapping means using a special process to perform a task that one
would be unable to do in general
– for example, writing a compiler in terms of itself
– start with a simple compiler, compile a more complex version in that
one
■ When write a Quine
– The program has a code part and a data part
– The data part represents the entire code
– But the code contains the data part
– Seems like a paradox: how can the data part contain itself?
– Bootstrapping needed:
• put special character in data part
• replace that character by the data part itself
23

Quine (C) 1/6
/* A simple quine (self-printing program), in standard C. */
/* Note: in designing this quine, we have tried to make the code clear
* and readable, not concise and obscure as many quines are, so that
* the general principle can be made clear at the expense of length.
* In a nutshell: use the same data structure (called "progdata"
* below) to output the program code (which it represents) and its own
* textual representation. */
#include <stdio.h>
void quote(const char *s)
/* This function takes a character string s and prints the
* textual representation of s as it might appear formatted
* in C code. */
{
int i;
printf(" "");
...
24

Quine (C) 2/6
for (i=0; s[i]; ++i) {
/* Certain characters are quoted. */
if (s[i] == '')
printf("");
else if (s[i] == '"')
printf(""");
else if (s[i] == 'n')
printf("n");
/* Others are just printed as such. */
else
printf("%c", s[i]);
/* Also insert occasional line breaks. */
if (i % 48 == 47)
printf(""n "");
}
printf(""");
}
...
25

Quine (C) 3/6
/* What follows is a string representation of the program code,
* from beginning to end (formatted as per the quote() function
* above), except that the string _itself_ is coded as two
* consecutive '@' characters. */
const char progdata[] =
"/* A simple quine (self-printing program), in st"
"andard C. */nn/* Note: in designing this quine, "
"we have tried to make the code clearn * and read"
"able, not concise and obscure as many quines are"
", so thatn * the general principle can be made c"
"lear at the expense of length.n * In a nutshell:"
" use the same data structure (called "progdata"n"
" * below) to output the program code (which it r"
"epresents) and its ownn * textual representation"
". */nn#include <stdio.h>nnvoid quote(const char "
"*s)n /* This function takes a character stri"
"ng s and prints then * textual representati"
"on of s as it might appear formattedn * in "
"C code. */n{n int i;nn printf(" "");n
...
26

Quine (C) 4/6
" for (i=0; s[i]; ++i) {n /* Certain cha"
"racters are quoted. */n if (s[i] == '')"
"n printf("");n else if (s["
"i] == '"')n printf(""");n e"
"lse if (s[i] == 'n')n printf("n");"
"n /* Others are just printed as such. */n"
" elsen printf("%c", s[i]);n "
" /* Also insert occasional line breaks. */n "
" if (i % 48 == 47)n printf("""
"n "");n }n printf(""");n}nn/* What fo"
"llows is a string representation of the program "
"code,n * from beginning to end (formatted as per"
" the quote() functionn * above), except that the"
" string _itself_ is coded as twon * consecutive "
"'@' characters. */nconst char progdata[] =n@@;nn"
"int main(void)n /* The program itself... */n"
"{n int i;nn /* Print the program code, cha"
"racter by character. */n for (i=0; progdata[i"
"]; ++i) {n if (progdata[i] == '@' && prog"
... 27

Quine (C) 5/6
"data[i+1] == '@')n /* We encounter tw"
"o '@' signs, so we must print the quotedn "
" * form of the program code. */n {n "
" quote(progdata); /* Quote all. */n"
" i++; /* Skip second '"
"@'. */n } elsen printf("%c", p"
"rogdata[i]); /* Print character. */n }n r"
"eturn 0;n}n";
int main(void)
/* The program itself... */
{
int i;
/* Print the program code, character by character. */
for (i=0; progdata[i]; ++i) {
if (progdata[i] == '@' && progdata[i+1] == '@')
/* We encounter two '@' signs, so we must print the quoted
* form of the program code. */
...
28
look for two consecutive
@ characters and replace
them by program string
itself

Quine (C) 6/6
{
quote(progdata); /* Quote all. */
i++; /* Skip second '@'. */
} else
printf("%c", progdata[i]); /* Print character. */
}
return 0;
}
■ Reflective questions (pun intended)
– Is this a metaprogram?
– Is it a reflective program?
– Is it self-modifying?
29

Wrap up
LSINF 2335
Prof. Kim Mens

Things to remember
■ Meta (programming)
■ (Computational) Reflection
■ Bootstrapping
31

Theory,Pract.andApplic. Comprehension exercises
■ Do you believe the Safir-Whorf hypothesis applies to
programming ?
■ Write a paradoxical reflective sentence and explain why it is
paradoxical
■ Is the following “definition” reflective?
– recursive: adjective, see recursive
■ Explain reflection, metaprogramming and bootstrapping in your own
words.
■ Explain the word “meta” without going meta.
■ Write a quine in a programming language of your choice.
■ Can you write one that makes use of reflection?
■ Are you programming reflectively when you use the keyword “this”
in Java?
■ Read the book : “Gödel, Escher, Bach”.
32

A gentle introduction to reflection

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