Magic (programming)

Last updated April 23, 2024

In the context of computer programming, magic is an informal term for abstraction; it is used to describe code that handles complex tasks while hiding that complexity to present a simple interface. The term is somewhat tongue-in-cheek, and often carries bad connotations, implying that the true behavior of the code is not immediately apparent. For example, Perl's polymorphic typing and closure mechanisms are often called "magic". The term implies that the hidden complexity is at least in principle understandable, in contrast to black magic and deep magic (see Variants), which describe arcane techniques that are deliberately hidden or extremely difficult to understand. However, the term can also be applied endearingly, suggesting a "charm" about the code. The action of such abstractions is described as being done "automagically", a portmanteau of "automatically" and "magically".

Referential opacity

"Magic" refers to procedures which make calculations based on data not clearly provided to them, by accessing other modules, memory positions or global variables that they are not supposed to (in other words, they are not referentially transparent). According to most recent software architecture models, even when using structured programming, it is usually preferred to make each function behave the same way every time the same arguments are passed to it, thereby following one of the basic principles of functional programming. When a function breaks this rule, it is often said to contain "magic".

A simplified example of negative magic is the following code in PHP:

functionmagic(){global$somevariable;echo$somevariable;}$somevariable=true;magic();

While the code above is clear, if it is seen in a large project, it is often hard to understand where the function magic() gets its value from. It is preferred to write that code using the following concept:

functionnoMagic($myvariable){echo$myvariable;}$somevariable=true;noMagic($somevariable);

Non-orthogonality

Any SV [scalar value] may be magical, that is, it has special features that a normal SV does not have.
— Larry Wall, perlguts manual page,^[1] Perl 5

This definition of magic or magical can be extended to a data type, code fragment, keyword, or machine address that has properties not shared by otherwise identical objects. The magical properties may or may not be documented.

In ISO C, file handles (of type FILE) cannot be safely copied as their addresses^[2] may be magic. That is, the runtime environment may place original file handles in a hard-coded address range, and not provide file handle behaviour to a user-created copy at another address. Consequently, the standard library routines accept pointers to file handles, of type FILE *, instead.
In Perl 5, the statement while(<file_handle>) implicitly assigns the line read from the file by <file_handle> to the variable $_, and applies the defined() function to the expression so that any successfully read string, even "0" or the empty string, evaluates as true and continues the while() loop. This does not happen to <file_handle> anywhere else, or to while() with any other control expression.^[3]
In an emulator, especially one in development, the emulated machine's system call points may be magic; when they are called, the emulator may run native code for convenience, speed or access to physical hardware, and set up the emulated CPU and memory as if it had executed the original code.
- For instance, the CALL statement of BBC BASIC V treats the system call addresses of Acorn MOS magically; instead of attempting to branch to ARM code at those addresses, it raises a software interrupt in RISC OS equivalent to the system call.^[4]^{[ non-primary source needed ]} The effect is to emulate Acorn MOS sufficiently for 8-bit BASIC programs not containing assembly language to run without modification.
Also in BBC BASIC, not only does the numeric variable @% control print formatting, it accepts direct assignment of ANSI printf format strings, normally a type mismatch error.^[4]
In JavaScript, evaluation of the typeof operator succeeds when the operand is an undeclared identifier, which would normally result in a ReferenceError.
Any comment that has an effect on the code is magic.
Memory-mapped I/O addresses and volatile variables are also magic in this sense, although the term is not normally applied.

Variants

Deep magic refers to techniques that are not widely known, and may be deliberately kept secret. The number of such techniques has arguably decreased over time, especially in the field of cryptography, where security through obscurity has been increasingly abandoned in favour of security through design which allows, and often encourages, public scrutiny.

The Jargon File makes a distinction between deep magic, which refers to code based on esoteric theoretical knowledge, and black magic, which refers to code based on techniques that appear to work but which lack a theoretical explanation.^[5] It also defines heavy wizardry, which refers to code based on obscure or undocumented intricacies of particular hardware or software.‌ For an example, the infamous “You are not expected to understand this” comment: many readers of Lions’ Book, a didactically annotated commentary on the Unix version 6 kernel source code, interpreted this comment, found in the code, to suggest some deep theoretical wisdom was on display in the obscure snippet, but in fact its "magical" function was merely dependant on a peculiarity of the way the C compiler was optimised for the PDP-11 hardware on which version 6 of Unix was designed to run.

Related Research Articles

C is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted CPUs. It has found lasting use in operating systems, device drivers, and protocol stacks, but its use in application software has been decreasing. C is commonly used on computer architectures that range from the largest supercomputers to the smallest microcontrollers and embedded systems.

Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language".

In computer programming, a string is traditionally a sequence of characters, either as a literal constant or as some kind of variable. The latter may allow its elements to be mutated and the length changed, or it may be fixed. A string is generally considered as a data type and is often implemented as an array data structure of bytes that stores a sequence of elements, typically characters, using some character encoding. String may also denote more general arrays or other sequence data types and structures.

In computing, serialization is the process of translating a data structure or object state into a format that can be stored or transmitted and reconstructed later. When the resulting series of bits is reread according to the serialization format, it can be used to create a semantically identical clone of the original object. For many complex objects, such as those that make extensive use of references, this process is not straightforward. Serialization of object-oriented objects does not include any of their associated methods with which they were previously linked.

In computing, a segmentation fault or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricted area of memory. On standard x86 computers, this is a form of general protection fault. The operating system kernel will, in response, usually perform some corrective action, generally passing the fault on to the offending process by sending the process a signal. Processes can in some cases install a custom signal handler, allowing them to recover on their own, but otherwise the OS default signal handler is used, generally causing abnormal termination of the process, and sometimes a core dump.

In computer science, control flow is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language.

In computer science, an interpreter is a computer program that directly executes instructions written in a programming or scripting language, without requiring them previously to have been compiled into a machine language program. An interpreter generally uses one of the following strategies for program execution:

Parse the source code and perform its behavior directly;
Translate source code into some efficient intermediate representation or object code and immediately execute that;
Explicitly execute stored precompiled bytecode made by a compiler and matched with the interpreter's Virtual Machine.

In computer programming, a reference is a value that enables a program to indirectly access a particular datum, such as a variable's value or a record, in the computer's memory or in some other storage device. The reference is said to refer to the datum, and accessing the datum is called dereferencing the reference. A reference is distinct from the datum itself.

In computer science, primitive data types are a set of basic data types from which all other data types are constructed. Specifically it often refers to the limited set of data representations in use by a particular processor, which all compiled programs must use. Most processors support a similar set of primitive data types, although the specific representations vary. More generally, "primitive data types" may refer to the standard data types built into a programming language. Data types which are not primitive are referred to as derived or composite.

Execution in computer and software engineering is the process by which a computer or virtual machine interpret and acts on the instructions of a computer program. Each instruction of a program is a description of a particular action which must be carried out, in order for a specific problem to be solved. Execution involves repeatedly following a "fetch–decode–execute" cycle for each instruction done by control unit. As the executing machine follows the instructions, specific effects are produced in accordance with the semantics of those instructions.

The Enterprise is a Zilog Z80-based home computer announced in 1983, but due to a series of delays, was not commercially available until 1985. It was developed by British company Intelligent Software and marketed by Enterprise Computers.

TI-BASIC is the official name of a BASIC-like language built into Texas Instruments (TI)'s graphing calculators. TI-BASIC is a language family of three different and incompatible versions, released on different products:

In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recursion is particularly useful, and is often easy to optimize in implementations.

A bit array is an array data structure that compactly stores bits. It can be used to implement a simple set data structure. A bit array is effective at exploiting bit-level parallelism in hardware to perform operations quickly. A typical bit array stores kw bits, where w is the number of bits in the unit of storage, such as a byte or word, and k is some nonnegative integer. If w does not divide the number of bits to be stored, some space is wasted due to internal fragmentation.

In computing, a data segment is a portion of an object file or the corresponding address space of a program that contains initialized static variables, that is, global variables and static local variables. The size of this segment is determined by the size of the values in the program's source code, and does not change at run time.

In computer science, a local variable is a variable that is given local scope. A local variable reference in the function or block in which it is declared overrides the same variable name in the larger scope. In programming languages with only two levels of visibility, local variables are contrasted with global variables. On the other hand, many ALGOL-derived languages allow any number of nested levels of visibility, with private variables, functions, constants and types hidden within them, either by nested blocks or nested functions. Local variables are fundamental to procedural programming, and more generally modular programming: variables of local scope are used to avoid issues with side-effects that can occur with global variables.

A webform, web form or HTML form on a web page allows a user to enter data that is sent to a server for processing. Forms can resemble paper or database forms because web users fill out the forms using checkboxes, radio buttons, or text fields. For example, forms can be used to enter shipping or credit card data to order a product, or can be used to retrieve search results from a search engine.

The null coalescing operator is a binary operator that is part of the syntax for a basic conditional expression in several programming languages, such as : C# since version 2.0, Dart since version 1.12.0, PHP since version 7.0.0., Perl since version 5.10 as logical defined-or, PowerShell since 7.0.0, and Swift as nil-coalescing operator.

In computing, an emulator is hardware or software that enables one computer system to behave like another computer system. An emulator typically enables the host system to run software or use peripheral devices designed for the guest system. Emulation refers to the ability of a computer program in an electronic device to emulate another program or device.

Charm is a computer programming language devised in the early 1990s with similarities to the RTL/2, Pascal and C languages in addition to containing some unique features of its own. The Charm language is defined by a context-free grammar amenable to being processed by recursive descent parser as described in seminal books on compiler design.

References

↑ "perlguts – perldoc.perl.org". 5 October 2014. Retrieved 18 February 2015.
↑ Banahan, Mike; Brady, Declan; Doran, Mark (1991). "9.10.3 The stdio.h header file". The C book: Featuring the ANSI C standard. The Instruction Set (2nd ed.). Wokingham, England: Addison-Wesley Publishers. p. 234. ISBN 0-201-54433-4. It is not safe to copy these objects within the program; sometimes their addresses may be 'magic'.
↑ "perlop – perldoc.perl.org". 7 September 2010. Retrieved 17 February 2011.
1 2 "27. Keywords" (PDF). BBC BASIC Reference Manual (1st ed.). Cambridge, England: Acorn Computers. October 1992. pp. 229, 349. ISBN 1-85250-103-0 . Retrieved 9 May 2007.^{[ dead link ]}
↑ "Deep Magic". Jargon File.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[perlguts-1] "perlguts – perldoc.perl.org". 5 October 2014. Retrieved 18 February 2015.

[2] Banahan, Mike; Brady, Declan; Doran, Mark (1991). "9.10.3 The stdio.h header file". The C book: Featuring the ANSI C standard. The Instruction Set (2nd ed.). Wokingham, England: Addison-Wesley Publishers. p. 234. ISBN 0-201-54433-4. It is not safe to copy these objects within the program; sometimes their addresses may be 'magic'.

[perlop-3] "perlop – perldoc.perl.org". 7 September 2010. Retrieved 17 February 2011.

[bbcbasic-4] 1 2 "27. Keywords" (PDF). BBC BASIC Reference Manual (1st ed.). Cambridge, England: Acorn Computers. October 1992. pp. 229, 349. ISBN 1-85250-103-0 . Retrieved 9 May 2007.^{[ dead link ]}

[5] "Deep Magic". Jargon File.

[1]

[2]

[3]

[4]

[5]