Have you ever met a prefix like this? (with some self-reference)

Have you ever met a (i.e., "meta") prefix like "meta"?

This is an interesting part of self-reference. In the field of computer science (and related fields) one often hears the term "meta" used as a prefix to other words.

• meta-data
• meta-table
• meta-language
• meta-programming
• meta-character
• ... and so on ...

The term meta, as a prefix, often means something that refers to itself or something like itself.

The modern Greek word "μετά" (meh-TA) ≈ "after" from which we get the English prefix "meta", although there is some history that has changed the meaning in English.

The modern Greek word "μεταμόρφωσης" (meh-ta-MOR-fo-sees) ≈ "metamorphosis" comes from the modern Greek word "μετά" and the modern Greek word "μορφή" (mor-FEE) ≈ "form".

The word literally means an "after form" that was, somehow, distinct. Over time, "after form" took on the connotation of a "changed form".

The modern Greek word "μετάνοια" (meh-TA-nee-a) ≈ "repentance" comes from the modern Greek word "μετά" and the modern Greek word "νους" (nus) ≈ "mind".

The word literally means an "after mind" or "after thought" as in "think this over in your mind". Over time, "after thought" took on the connotation of a "changed mind" or "changed thought".

This re-thinking took on the meaning that in English is "repent". When you "repent", do you do a "pent" again? Do you do "penance" again? Does the first time count?

If the prefix "re" means "again", then I will have to "research" that (for the very first time).

• Meta-data is data about data.
• SQL meta-data is a description of data tables (including itself).
• A meta-table is a table that describes a table.

One advantage of representing meta-data like data is that you can do searches, queries, etc., of the meta-data using the same tools and techniques that you use to search and query the data.

A meta-character is a character that describes or represents another character. Usually, that character is used for special purposes so that a character that is different than that character needs to be used.

Sometimes an escape character is used that precedes the character that is to be given special treatment.

The term escape character and meta-character are sometimes used interchangeably.

Here are a few common meta-character escape sequences used in many languages.

• "\t" is the tab character
• "\n" is the newline character
• "\r" is the return character
• "\\" is the backslash character

Meta-characters are crucial in self-referential programs such as programs that write programs and programs that replicate themselves (in analogy to DNA replication).

A meta-dream is a dream about a dream.

A meta-joke is a joke about a joke. People who like to tell a bad joke have never met a joke that they do not like.

What is a meta-joker?

A meta-meeting is a meeting about a meeting. One popular Dilbert cartoon has a pre-planning meeting for the planning meeting for the meeting.

Meta-data is data about data.

The meta-data of a phone conversation (in security terms sometimes called traffic analysis) includes the fact that you made a call, who you called, when you called, how long the call lasted, etc., but not the actual contents of your phone conversation.

A meta-compiler is compiler that compiles (or helps write) a compiler, called a compiler-compiler.

A popular Linux-based meta-compiler or compiler-compiler is YACC (Yet Another Compiler Compiler) or an improved version called Bison (a play on the animal name yac as being pronounced similarly to YACC).

A meta-program is a program that writes a program, as in a meta-compiler.

In computer science, the term reflection refers to using a program to look at it's own code in a meta-programming way.

Some languages that have good reflection support: C#, Python, Julia and (of course) LISP (and variants).

The DNA code can replicate itself. It has reflection capabilities (e.g., AIDS virus as in a rootkit-viris).

The field of programming languages in computer science is concerned, in part, with programs that write programs.

For some simple examples in C, see Programs that write programs.

A meta-language is a language that describes a language.

An English class can use English to describe the syntax and semantics of the English language.

Whenever I study a language (German, Greek, Russian, etc.) I enjoy getting to the point where I can start learning more of the language by reading books in that language that talk about the language.

• Dictionary of terms of that language defined in that language.
• Etymology of terms in the language written in that language.

To learn, say, some Russian, it is useful, knowing German and Greek, to read books in German and Greek that teach Russian - reinforcing the other languages while learning a new language.

The C++ programming language has a Turing-complete meta-programming system that is an extension and improvement of the C directive preprocessor that has directives such as include, define, ifdef, etc.

• A meta-design is an design about how to design.
• A meta-analysis is an analysis about ways to analyze.
• A meta-policy is a policy about policies.
• A meta-decision is a decision about decisions.
• A meta-process is a process about processes (including itself)
• ... and so on ...

A meta-process is a process about processes.

Actual event: Department CS meeting about making sure we cover top-down design and programming in a course, an important concept in CS, despite not using it other places in the course nor in solving actual problems faced by the CS department.

A meta-design is a design about designs. Book: The design of design by Frederick Brooks, Jr..

Note: Frederick Brooks, Jr., is the author of "The mythical man month", an important book in the history of software engineering principles. Today, the term used would be a "person month" rather than a "man month".

An XML DTD (Document Type Definition) is a meta-language for describing XML schema.

A DTD is somewhat hard to work with. For one company project, I ended up representing the DTD grammar as EBNF, using the Lua PEG (Parsing Expression Grammar) to recognize

Almost every programming language has a grammar that has been expressed in EBNF. For example, search the Internet for the following.

Here is a (simplified) grammar of C statements as covered in most introductory programming courses and expressed as an EBNF syntax diagram (ignoring some statements as goto, switch, etc.)

Here is the textual EBNF grammar of the above syntax diagrams.

Note: The full grammar precisely defines where semicolons are needed and where they are not needed and where they are optional.

Some acronyms are meta-acronyms or otherwise recursive.

• A TLA (Three Letter Acronym) is a TLA. That is, the definition includes itself. This is also known as "autological".

A common phrase in the GNT (Greek New Testament) is "μετά ταύτα" (meh-TA TAV-ta) ≈ "after these things". John uses this exact phrase 8 times in Revelation and 7 times in John. It appears 8 other times in the GNT (but not in 1, 2, or 3 John).

The modern Greek word "μετά" (meh-TA) ≈ "after" is used in the modern Greek word "μεταφυσική" (meh-ta-fee-see-KEE) ≈ "metaphysics" which is what comes "after physics", literally, the book Aristotle wrote after his book on physics.

The modern Greek word "ταύτα" (TAV-ta) ≈ "these things" is a contraction of the Greek phrase "το αυτό" (to av-TO) ≈ "the self" (singular) and, in this case "τα αυτά" (ta av-TA) ≈ "these" (plural).

From these, we get the word "ταυτολογία" (tav-to-lo-Y-a) ≈ "tautology", through Latin "tautologia" ≈ "tautology", which is a representation of the same thing using other words. In mathematics, a "tautology" is something that is always true (in and of itself, without other considerations).

For example, the mathematical formula

means that "x" (whatever "x" is), is equal to "x" and is always "true" (whatever "true" means). Note that in computer programming languages such as C#, Java, JavaScript, etc. the statement

means take the value of "x" and assign it to itself (a side-effect that changes the state of the computer). On the other hand, the expression

is the true value of the expression which is always true in that "x" is always equal to itself. It is a tautology and can therefore be simplified to the expression

where true is always true and false is always false and they are not the same (they are opposites in the sense of the operation of set compliment).

One way to identify a beginning programmer is that they would write something like

instead of

which can be simplified to the following.

Note that an optimizing DNA compiler might recognize and do this (and many other) optimizations making the final DNA code difficult to abstract to a higher level language notation (as used by human programmers).