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It’s explained here: <a href="http://erlang.org/doc/reference_manual/expressions.html#id83710">http://erlang.org/doc/reference_manual/expressions.html#id83710</a>
</div><div><span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; line-height: normal; widows: 1;">The set of valid </span><strong style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; line-height: normal; widows: 1;">guard expressions</strong><span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; line-height: normal; widows: 1;"> (sometimes called guard tests) is a subset of the set of valid Erlang expressions. The reason for restricting the set of valid expressions is that evaluation of a guard expression must be guaranteed to be free of side effects.</span></div><div>
<p style="font-family: Helvetica; font-size: 10pt; margin-bottom: 0cm;">
</p><p style="font-family: Helvetica; font-size: 10pt; margin-bottom: 0cm;">
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</p><p style="margin-bottom: 0cm"><font color="#000000"><font face="Helvetica, sans-serif"><font style="font-size: 9pt">Kind
regards,
<br>Radek
Gruchalski<br>
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<p style="color: #A0A0A8;">On Friday, 5 February 2016 at 13:40, Theepan wrote:</p>
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<span><div><div><div dir="ltr"><p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">Team - Any idea?</p><p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo"><br></p><p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">60> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">60> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">60> is_integer(A = 5). </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">true</p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">61> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">61> fun () when is_integer(A = 5) -> ok end. </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">* 2: illegal guard expression</p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">62> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">62> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">62> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">62> fun () when is_integer(5) -> ok end. </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">#Fun<erl_eval.20.54118792></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">63> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">63> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">63> fun (A = 5) when is_integer(A) -> ok end. </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">#Fun<erl_eval.6.54118792></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">64> </p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo">64> </p><div><br></div></div>
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</blockquote>
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