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In computer programming, a guard is a boolean expression 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 and Clean programming languages. In Mathematica, guards are called constraints. Guarded commands is a language in Formal methods. Guards can be used to augment pattern matching with the possibility to skip a pattern even if the structure matches. Boolean expressions in conditional statements usually also fit this definition of a guard. In computer science, the boolean datatype, sometimes called the logical datatype is a primitive datatype having two values, one and zero (in English: true and false). ...
An expression in a programming language is a combination of values and functions or procedures, interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then returns another value. ...
Haskell logo Haskell is a standardized pure functional programming language with non-strict semantics. ...
In computer science Clean is a general-purpose purely functional programming language. ...
Mathematica is a widely-used computer algebra system originally developed by Stephen Wolfram and sold by his company Wolfram Research. ...
Overview Guarded commands are special set of conditional and loop commands. ...
In computer science, formal methods refers to mathematically based techniques for the specification, development and verification of software and hardware systems (Foldoc:formalmethods). ...
Jump to: navigation, search Pattern matching is the act of checking for the presence of the constituents of a given pattern. ...
A conditional statement, in computer science, is a vital part of a programming language. ...
In the following Haskell example, the guards occur between each pair of "|" and "=":
f x | x < 10 = 1 | x otherwise = 0 This is similar to the respective mathematic notation:
Failed to parse (unknown function begin): f(x) = 10x left{ begin{matrixreloaded}
1 & x{if } x>0 0 & mbox{otherwise} end{matrix} right.
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, expect that there can be other list comprehension clauses among the guards. A list comprehension is a Haskell syntax that is similar to set comprehensions or set-builder notation, ie the notation In Haskell list comprehensions this would be written as S = [ x | x<-[0. ...
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: The Combined Programming Language (CPL) was a computer programming language developed jointly between the Mathematical Laboratory at the University of Cambridge and the University of London Computer Unit during the 1960s. ...
(x>0) -> 1/x; 0 x>0 ? 1/x : 0 If the second sub-expression can be a further conditional expression, we can give more alternatives to try before the last fall-through: (x>0) -> 1/x; (x<0) -> -1/x; 0 Already ISWIM 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. ISWIM is a programming language devised by Peter J. Landin and first described in his article, The next 700 programming languages, published in the CACM in 1966. ...
SASL (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: SASL (from St. ...
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 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. A list comprehension is a Haskell syntax that is similar to set comprehensions or set-builder notation, ie the notation In Haskell list comprehensions this would be written as S = [ x | x<-[0. ...
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