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    <p><strong>The LAMBDA expression</strong></p> <p><code>(lambda ...)</code> is considered to be a <em>lambda expression</em> only in certain places, like the <code>function</code> form or as the head of a function call. Lambda expressions are not evaluated.</p> <pre><code>(function ; special operator FUNCTION (lambda () 'foobar)) ; &lt;- this is a lambda expression ( ; a function call (lambda (foo) foo) ; &lt;- this is a lambda expression 'bar ; argument ) </code></pre> <p>But here <code>(lambda ...)</code> is a macro form and not a lambda expression:</p> <pre><code>(funcall ; calling a function via funcall (lambda (foo) foo) ; this is not a lambda expressions, but the macro lambda ; as all arguments to FUNCALL it will be ; macro expanded and evaluated ; it expands to (function (lambda (foo) foo)) 'bar) ; argument </code></pre> <p><strong>The LAMBDA macro</strong></p> <p><a href="http://www.lispworks.com/documentation/HyperSpec/Body/m_lambda.htm" rel="nofollow noreferrer">LAMBDA</a> is a macro. It expands <code>(lambda ...)</code> to <code>(function (lambda ...))</code>, which is the equivalent of <code>#'(lambda ...))</code>.</p> <pre><code>CL-USER &gt; (macroexpand '(lambda (foo) foo)) (FUNCTION (LAMBDA (FOO) FOO)) </code></pre> <p>The macro saves you a bit of writing/reading, that's all. In the first version of Common Lisp (CLtL1) there was no <code>LAMBDA</code> macro. It has been added later and is now a part of <em>ANSI Common Lisp</em>,</p> <p><strong>The FUNCTION special operator</strong></p> <p><a href="http://www.lispworks.com/documentation/HyperSpec/Body/s_fn.htm#function" rel="nofollow noreferrer">FUNCTION</a> is a special operator. It expects a function name or a <em>lambda expression</em>. Thus the name or the <em>lambda expression</em> are not evaluated. In fact <em>lambda expressions</em> can't be evaluated at all. Inside <code>FUNCTION</code>, the <em>lambda expression</em> is <strong>not</strong> a macro form and thus will <strong>not</strong> be expanded again. The purpose of <code>FUNCTION</code> is to return the corresponding function object which is denoted by the name or by the <em>lambda expression</em>. It returns the function object as a value. With this special operator one can access the function object from global functions and lexical functions.</p> <p>The <code>FUNCTION</code> operator is necessary in Common Lisp, because it has separate namespaces for values, functions and a few other things. It as a so-called <em>Lisp-2</em> or even <em>Lisp-n</em>, with two or more namespaces.</p> <p><strong>Lambda expressions in function position in a function form</strong></p> <p><code>((lambda (foo) foo) 10)</code> is supported by built-in syntax for Common Lisp. See <a href="http://www.lispworks.com/documentation/HyperSpec/Body/03_ababd.htm" rel="nofollow noreferrer">Lambda Forms</a>.</p> <p><strong>Confusing</strong></p> <p>This is all logical, but confusing. Don't worry you are not alone, but in practice it's not a big deal.</p>
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    1. USRainer Joswig
    1. USRainer Joswig
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    1. COThanks, I still don't get why people explicitly write #' before a lambda expression though.
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      1. USturingcomplete
    2. COIt's a matter of style. Some prefer it, some don't. I prefer it, as it allows to easily identify actual functions used as values. It also makes it easier to replace with an `flet` or `labels` function. Other than lambdas, it also makes a function's definition be looked up. This may be desirable if you don't know if an implementation coerces a [function designator](http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_f.htm#function_designator) upfront, e.g. `(mapcar 'frob my-list)` may lookup `frob`'s `symbol-function` as many times as the elements in `my-list`.
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      1. USacelent
    3. CODoug Hoyte provides an argument for not using the #' syntax. Rainer hints at it here; the fact that ((lambda (foo) is built-in syntax, and there's a (lambda macro, means that you can do some interesting things with this duality of syntax when using read macros. Check out Hoyte's cl-ppcre read-macro extension as an example: http://letoverlambda.com/index.cl/guest/chap4.html#sec_4
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      1. USClayton Stanley
 

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