Syntactic sugar
In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an alternative style that some may prefer.
For example, many programming languages provide special syntax for referencing and updating array elements. Abstractly, an array reference is a procedure of two arguments: an array and a subscript vector, which could be expressed as
get_array. Instead, many languages provide syntax such as Array. Similarly an array element update is a procedure consisting of three arguments, for example set_array, but many languages provide syntax such as Array = value.A construct in a language is called "syntactic sugar" if it can be removed from the language without any effect on what the language can do: functionality and expressive power will remain the same.
Language processors, including compilers and static analyzers, often expand sugared constructs into more fundamental constructs before processing, a process sometimes called "desugaring".
Origins
The term syntactic sugar was coined by Peter J. Landin in 1964 to describe the surface syntax of a simple ALGOL-like programming language which was defined semantically in terms of the applicative expressions of lambda calculus, centered on lexically replacing λ with "where".Later programming languages, such as CLU, ML and Scheme, extended the term to refer to syntax within a language which could be defined in terms of a language core of essential constructs; the convenient, higher-level features could be "desugared" and decomposed into that subset. This is, in fact, the usual mathematical practice of building up from primitives.
Building on Landin's distinction between essential language constructs and syntactic sugar, in 1991, Matthias Felleisen proposed a codification of "expressive power" to align with "widely held beliefs" in the literature. He defined "more expressive" to mean that without the language constructs in question, a program would have to be completely reorganized.
Notable examples
- In COBOL, many of the intermediate keywords are syntactic sugar that may optionally be omitted. For example, the sentence
MOVE A B.and the sentenceMOVE A TO B.perform exactly the same function, but the second makes the action to be performed clearer. - Augmented assignment or compound assignment operators: For example,
a += bis equivalent toa = a + bin C and similar languages, assumingahas no side effects such as ifais a regular variable. - In Perl,
unlessis syntactic sugar forif. Additionally, any statement can be followed by a condition, sostatement if conditionis equivalent toif, but the former is more naturally formatted on a single line. - In the C language, the
anotation is syntactic sugar for*. Likewise, thea->xnotation is syntactic sugar for accessing members using the dereference operator.x. - The
usingstatement in C# ensures that certain objects are disposed of correctly. The compiler expands the statement into a try-finally block. - The C# language allows variables to be declared as
var x = expr, which allows the compiler to infer the type ofxfrom the expressionexpr, instead of requiring an explicit type declaration. Similarly, C++ allowsauto x = exprsince C++11. - Python list comprehensions and decorators.
- In Haskell, a string, denoted in quotation marks, is semantically equivalent to a list of characters.
- In the tidyverse collection of R packages, the pipe, denoted by
%>%, declares that the data preceding the pipe will serve as the first argument for the function following the pipe. So,x %>% fis equivalent tof. - In SQL,
JOINis equivalent toINNER JOIN, the latter clarifies that the join statement is specifically an inner join operation as opposed to an outer join operation. - Method calling in OOP languages in the form of
myObject.myMethodis syntactic sugar for calling global function asmyMethod. The reference to the object is passed as a hidden argument, usually accessible from within the method asthis. - Parameters called by reference is a syntax sugar for technically passing a pointer to the parameter, but syntactically handling it as the variable itself, to avoid constant de-referencing of pointer in the code inside the function.
Criticism
For example, Alan Perlis once quipped in "Epigrams on Programming", in a reference to bracket-delimited languages, that "Syntactic sugar causes cancer of the semi-colons".
Derivative terms
Syntactic salt
The metaphor has been extended by coining the term syntactic salt, which indicates a feature designed to make it harder to write bad code. Specifically, syntactic salt is a hoop that programmers must jump through just to prove that they know what is going on, rather than to express a program action. For example, in Java and Pascal assigning a float value to a variable declared as an int without additional syntax explicitly stating that intention will result in a compile error, while C and C++ will automatically truncate any floats assigned to an int. However this is not syntax, but semantics.In C#, when hiding an inherited class member, a compiler warning is issued unless the
new keyword is used to specify that the hiding is intentional. To avoid potential bugs owing to the similarity of the switch statement syntax with that of C or C++, C# requires a break for each non-empty case label of a switch even though it does not allow implicit fall-through. Syntactic salt may defeat its purpose by making the code unreadable and thus worsen its quality – in extreme cases, the essential part of the code may be shorter than the overhead introduced to satisfy language requirements.
An alternative to syntactic salt is generating compiler warnings when there is high probability that the code is a result of a mistake – a practice common in modern C/C++ compilers.
Syntactic saccharin
Other extensions are syntactic saccharin and syntactic syrup, meaning gratuitous syntax that does not make programming any easier.Sugared types
Data types with core syntactic support are said to be "sugared types."Common examples include quote-delimited strings, curly braces for object and record types, and square brackets for Arrays.