In computer science, the Boolean or logical data type is a data type, having two values , intended to represent the truth values of logic and Boolean algebra...
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
that must evaluate to true if the program execution is to continue in the branch in question. The term is used at least in Haskell
Haskell (programming language)
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...
In computer science, Clean is a general-purpose purely functional computer programming language.-Features:The language Clean first appeared in 1987 and is still further developed; it shares many properties with Haskell:...
Erlang is a general-purpose concurrent, garbage-collected programming language and runtime system. The sequential subset of Erlang is a functional language, with strict evaluation, single assignment, and dynamic typing. For concurrency it follows the Actor model. It was designed by Ericsson to...
occam is a concurrent programming language that builds on the Communicating Sequential Processes process algebra, and shares many of its features. It is named after William of Ockham of Occam's Razor fame....
PROMELA is a verification modeling language. The language allows for the dynamic creation of concurrent processes to model, for example, distributed systems. In PROMELA models, communication via message channels can be defined to be synchronous , or asynchronous...
, OCaml and Scala programming languages. In Mathematica
Mathematica
Mathematica is a computational software program used in scientific, engineering, and mathematical fields and other areas of technical computing...
The Guarded Command Language is a language defined by Edsger Dijkstra for predicate transformer semantics. It combines programming concepts in a compact way, before the program is written in some practical programming language...
In computer science and software engineering, formal methods are a particular kind of mathematically-based techniques for the specification, development and verification of software and hardware systems...
In computer science, pattern matching is the act of checking some sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact. The patterns generally have the form of either sequences or tree structures...
with the possibility to skip a pattern even if the structure matches. Boolean expressions in conditional statement
Conditional statement
In computer science, conditional statements, conditional expressions and conditional constructs are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false...
s usually also fit this definition of a guard although they are called conditions.
In the following Haskell example, the guards occur between each pair of "|" and "=":
f x
| x > 0 = 1
| otherwise = 0
This is similar to the respective mathematical notation:
In this case the guards are in the "if" and "otherwise" clauses.
If there are several parallel guards, such as in the example above, they are normally tried in a top to bottom order and the branch of the first to pass is chosen. Guards in a list of cases are typically parallel.
However, in Haskell list comprehensions the guards are in series, and if any of them fails, the list element is not produced. This would be the same as combining the separate guards with logical AND
Logical conjunction
In logic and mathematics, a two-place logical operator and, also known as logical conjunction, results in true if both of its operands are true, otherwise the value of false....
, except that there can be other list comprehension clauses among the guards.
Evolution
A simple conditional expression, already present in CPL in 1963, has a guard on first sub-expression, and another sub-expression to use in case the first one cannot be used. Some common ways to write this:
(x>0) -> 1/x; 0
x>0 ? 1/x : 0
If the second sub-expression can be a further simple conditional expression, we can give more alternatives to try before the last fall-through:
(x>0) -> 1/x; (x<0) -> -1/x; 0
ISWIM is an abstract computer programming language devised by Peter J. Landin and first described in his article, The Next 700 Programming Languages, published in the Communications of the ACM in 1966...
in 1966 had a form of conditional expression without an obligatory fall-through case, thus separating guard from the concept of choosing either-or. In the case of ISWIM, if none of the alternatives could be used, the value was to be undefined, which was defined to never compute into a value.
SASL is a purely functional programming language developed by David Turner at the University of St Andrews in 1972, based on the applicative subset of ISWIM. In 1976 Turner redesigned and reimplemented it as a non-strict language...
(1976) was one of the first programming languages to use the term "guard". In the language, functions could have several definitions and the one to apply was chosen based on the guards that followed each definition:
fac n = 1, n = 0
= n * fac (n-1), n > 0
Pattern guard
In addition to a guard attached to a pattern, pattern guard can refer to the use of pattern matching in context of a guard. In effect, a match of the pattern is taken to mean pass. This meaning was introduced by a proposal for Haskell by Simon Peyton Jones
Simon Peyton Jones
Simon Peyton Jones is a British computer scientist who researches the implementation and applications of functional programming languages, particularly lazy functional languages...
titled A new view of guards in April 1997 and has been further used about the implementation of the proposal. The feature provides the possibility to use patterns in the guards of a pattern.
An example in the extended Haskell:
clunky env var1 var2
| Just val1 <- lookup env var1
, Just val2 <- lookup env var2
= val1 + val2
-- ...other equations for clunky...
This would read: "Clunky for an environment and two variables, in case that the lookups of the variables from the environment produce values, is the sum of the values. ..." Like in list comprehensions, the guards are in series, and if any of them fails the branch is not taken.
In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...
In computer programming, a switch, case, select or inspect statement is a type of selection control mechanism that exists in most imperative programming languages such as Pascal, Ada, C/C++, C#, Java, and so on. It is also included in several other types of languages...
In concurrent programming, guarded suspension is a software design pattern for managing operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed...
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