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Guard (computer science)

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In <a href="Computer_programming" title="Computer programming">computer programming</a>, a guard is a <a href="Boolean_expression" title="Boolean expression">Boolean expression</a> that must evaluate to true if the <a href="Execution_(computing)" title="Execution (computing)">execution</a> of the <a href="Computer_program" title="Computer program">program</a> is to continue in the branch in question. Regardless of which <a href="Programming_language" title="Programming language">programming language</a> is used, a guard clause, guard code, or guard statement is a check of integrity <a href="Precondition" title="Precondition">preconditions</a> used to avoid errors during execution.

The term guard clause is a <a href="Software_design_pattern" title="Software design pattern">Software design pattern</a> attributed to <a href="Kent_Beck" title="Kent Beck">Kent Beck</a> who codified many often unnamed coding practices into named software design patterns, the practice of using this technique dates back to at least the early 1960's. The guard clause most commonly is added at the beginning of a procedure and is said to "guard" the rest of the procedure by handling edgecases upfront.

A typical example is checking that a <a href="Reference_(computer_science)" title="Reference (computer science)">reference</a> about to be processed is not null, which avoids <a href="Null_pointer" title="Null pointer">null-pointer</a> failures.

Other uses include using a Boolean field for <a href="Idempotence" title="Idempotence">idempotence</a> (so subsequent calls are <a href="NOP_(code)" title="NOP (code)">nops</a>), as in the <a href="Dispose_pattern" title="Dispose pattern">dispose pattern</a>.<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public String foo(String username) {

if (username == null) {

throw new IllegalArgumentException("Username is null.");

}

// Rest of the method code follows here...

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Flatter_code_with_less_nesting">Flatter code with less nesting</h3></div>

The guard provides an <a href="Early_exit" class="mw-redirect" title="Early exit">early exit</a> from a <a href="Subroutine" class="mw-redirect" title="Subroutine">subroutine</a>, and is a commonly used deviation from <a href="Structured_programming" title="Structured programming">structured programming</a>, removing one level of nesting and resulting in flatter code:<a href="#cite_note-beck-1">[1]</a> replacing <code>if guard { ... }</code> with <code>if not guard: return; ...</code>.

Using guard clauses can be a <a href="Refactoring" class="mw-redirect" title="Refactoring">refactoring</a> technique to improve code. In general, less nesting is good, as it simplifies the code and reduces cognitive burden.

For example, in <a href="Python_(programming_language)" title="Python (programming language)">Python</a>:

<div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre># This function has no guard clause

def f_noguard(x):

if isinstance(x, int):

#code

return x + 1

else:

return None

# Equivalent function with a guard clause. Note that most of the code is less indented, which makes it easier to read and reason about

def f_guard(x):

if not isinstance(x, int):

return None

#code

return x + 1

</pre></div>

Another example, written in <a href="C_(programming_language)" title="C (programming language)">C</a>:

<div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre>// This function has no guard clause

int funcNoGuard(int x) {

if (x >= 0) {

//code

return x + 1;

} else {

return 0;

}

}

// Equivalent function with a guard clause

int funcGuard(int x) {

if (x < 0) {

return 0;

}

//code

}

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Terminology">Terminology</h2></div>

The term is used with specific meaning in <a href="APL_(programming_language)" title="APL (programming language)">APL</a>, <a href="Haskell_(programming_language)" class="mw-redirect" title="Haskell (programming language)">Haskell</a>, <a href="Clean_programming_language" class="mw-redirect" title="Clean programming language">Clean</a>, <a href="Erlang_programming_language" class="mw-redirect" title="Erlang programming language">Erlang</a>, <a href="Occam_(programming_language)" title="Occam (programming language)">occam</a>, <a href="Promela" title="Promela">Promela</a>, <a href="OCaml" title="OCaml">OCaml</a>, <a href="Swift_(programming_language)" title="Swift (programming language)">Swift</a>,<a href="#cite_note-2">[2]</a> <a href="Python_(programming_language)" title="Python (programming language)">Python</a> from version 3.10, and <a href="Scala_(programming_language)" title="Scala (programming language)">Scala</a> programming languages. In <a href="Mathematica" class="mw-redirect" title="Mathematica">Mathematica</a>, guards are called constraints. Guards are the fundamental concept in <a href="Guarded_Command_Language" title="Guarded Command Language">Guarded Command Language</a>, a language in <a href="Formal_methods" title="Formal methods">formal methods</a>. Guards can be used to augment <a href="Pattern_matching" title="Pattern matching">pattern matching</a> with the possibility to skip a pattern even if the structure matches. Boolean expressions in <a href="Conditional_(programming)" class="mw-redirect" title="Conditional (programming)">conditional statements</a> usually also fit this definition of a guard although they are called conditions.

<div class="mw-heading mw-heading2"><h2 id="Mathematics">Mathematics</h2></div>

In the following Haskell example, the guards occur between each pair of "|" and "=":

<div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre>f x

| x > 0 = 1

| otherwise = 0

</pre></div>

This is similar to the respective mathematical notation:

<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle f(x)=\left\{{\begin{matrix}1&{\mbox{if }}x>0\\0&{\mbox{otherwise}}\end{matrix}}\right.}">

<mrow>

<mtr>

<mtd>

</mtd>

<mtd>

</mstyle>

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<mtext>otherwise</mtext>

</mstyle>

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<annotation encoding="application/x-tex">{\displaystyle f(x)=\left\{{\begin{matrix}1&{\mbox{if }}x>0\\0&{\mbox{otherwise}}\end{matrix}}\right.}</annotation>

</semantics>

</math><img src="./62347de4b635ba2e610fcb648be075768c081346.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.505ex; width:23.073ex; height:6.176ex;" alt="{\displaystyle f(x)=\left\{{\begin{matrix}1&{\mbox{if }}x>0\\0&{\mbox{otherwise}}\end{matrix}}\right.}" loading="lazy">

In this case the guards are in the "if" and "otherwise" clauses.

<div class="mw-heading mw-heading2"><h2 id="Multiple_guards">Multiple guards</h2></div>

If there are several parallel guards, 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 <a href="List_comprehension" title="List comprehension">list comprehensions</a> 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 <a href="Logical_conjunction" title="Logical conjunction">logical AND</a>, except that there can be other list comprehension clauses among the guards.

<div class="mw-heading mw-heading2"><h2 id="Evolution">Evolution</h2></div>

A simple conditional expression, already present in <a href="CPL_programming_language" class="mw-redirect" title="CPL programming language">CPL</a> 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:

<pre>(x>0) -> 1/x; 0

x>0 ? 1/x : 0

</pre>

If the second sub-expression can be a further simple conditional expression, we can give more alternatives to try before the last fall-through:

<pre>(x>0) -> 1/x; (x<0) -> -1/x; 0

</pre>

In 1966 <a href="ISWIM" title="ISWIM">ISWIM</a> 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.

<a href="Kent_Recursive_Calculator" title="Kent Recursive Calculator">KRC</a>, a "miniaturized version"<a href="#cite_note-3">[3]</a> of <a href="SASL_programming_language" class="mw-redirect" title="SASL programming language">SASL</a> (1976), was one of the first programming languages to use the term "guard". Its function definitions could have several clauses, and the one to apply was chosen based on the guards that followed each clause:

<div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre> fac n = 1, n = 0

= n * fac (n-1), n > 0

</pre></div>

Use of guard clauses, and the term "guard clause", dates at least to <a href="Smalltalk" title="Smalltalk">Smalltalk</a> practice in the 1990s, as codified by <a href="Kent_Beck" title="Kent Beck">Kent Beck</a>.<a href="#cite_note-beck-1">[1]</a>

In 1996, Dyalog APL adopted an alternative pure functional style in which the guard is the only control structure.<a href="#cite_note-4">[4]</a> This example, in APL, computes the parity of the input number:<div class="mw-highlight mw-highlight-lang-apl mw-content-ltr" dir="ltr"><pre>parity←{

2∣⍵ : 'odd'

'even'

}

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Pattern_guard">Pattern guard</h2></div>

In addition to a guard attached to a pattern, pattern guard can refer to the use of <a href="Pattern_matching" title="Pattern matching">pattern matching</a> in the context of a guard. In effect, a match of the pattern is taken to mean pass. This meaning was introduced in a proposal for Haskell by <a href="Simon_Peyton_Jones" title="Simon Peyton Jones">Simon Peyton Jones</a> titled <a rel="nofollow" class="external text" href="https://web.archive.org/web/20160203203811/http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html">A new view of guards</a> in April 1997 and was used in the implementation of the proposal. The feature provides the ability to use patterns in the guards of a pattern.

An example in extended Haskell:

<div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre> clunky env var1 var2

| Just val1 <- lookup env var1

, Just val2 <- lookup env var2

= val1 + val2

-- ...other equations for clunky...

</pre></div>

This would read: "Clunky for an environment and two variables, in case the lookups of the variables from the environment produce values, is the sum of the values. ..." As in <a href="List_comprehension" title="List comprehension">list comprehensions</a>, the guards are in series, and if any of them fails the branch is not taken.

<ul><li><a href="Assertion_(computing)" class="mw-redirect" title="Assertion (computing)">Assertion</a></li>

<li><a href="Guarded_Command_Language" title="Guarded Command Language">Guarded Command Language</a>, a programming language based on non-deterministic conditionals</li>

<li><a href="Guarded_suspension" title="Guarded suspension">Guarded suspension</a></li>

<li><a href="Iverson_bracket" title="Iverson bracket">Iverson bracket</a></li>

<li><a href="Logical_conditional" class="mw-redirect" title="Logical conditional">Logical conditional</a></li>

<li><a href="Sentinel_node" title="Sentinel node">Sentinel node</a>, an object to represent the end of a data structure</li>

<li><a href="Switch_statement" title="Switch statement">Switch statement</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFBeck1997" class="citation book cs1"><a href="Kent_Beck" title="Kent Beck">Beck, Kent</a> (1997). "Guard Clause". Smalltalk Best Practice Patterns,. pp. 178–179.</cite>

</li>

<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite id="CITEREFCook" class="citation web cs1">Cook, Nate. <a rel="nofollow" class="external text" href="http://nshipster.com/guard-and-defer/">"guard & defer"</a>. NSHipster. Retrieved 2016-02-26.</cite>

</li>

<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite id="CITEREFTurner" class="citation web cs1">Turner, D. A. <a rel="nofollow" class="external text" href="https://www.cs.kent.ac.uk/people/staff/dat/tfp12/tfp12.pdf">"Some History of Functional Programming Languages"</a> (PDF).</cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite id="CITEREFScholes" class="citation web cs1">Scholes, John. <a rel="nofollow" class="external text" href="http://www.dyalog.com/uploads/documents/Papers/dfns.pdf">"Direct Functions in Dyalog APL"</a> (PDF).</cite>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://foldoc.org/guard">Guard</a> in Free On-Line Dictionary of Computing - FOLDOC, Denis Howe (editor).</li>

<li><a rel="nofollow" class="external text" href="http://c2.com/cgi/wiki?GuardClause">Guard Clause</a>, <a href="WikiWikiWeb" title="WikiWikiWeb">WikiWikiWeb</a></li>

<li>The Haskell 98 Report, chapter <a rel="nofollow" class="external text" href="http://haskell.org/onlinereport/exps.html">3 Expressions</a>.</li>

<li>The Mathematica Book, section <a rel="nofollow" class="external text" href="https://web.archive.org/web/20050408114902/http://documents.wolfram.com/mathematica/book/section-2.3.5">2.3.5 Putting Constraints on Patterns</a></li>

<li>The Glorious Glasgow Haskell Compilation System User's Guide, Version 6.4, section <a rel="nofollow" class="external text" href="https://web.archive.org/web/20051129140339/http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards">7.3.2. Pattern guards</a></li></ul></div><div><div class="zim-footer">

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