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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="sec-functions-library-asserts">
<title>Assert functions</title>
<section xml:id="function-library-lib.asserts.assertMsg">
<title><function>lib.asserts.assertMsg</function></title>
<subtitle><literal>assertMsg :: Bool -> String -> Bool</literal>
</subtitle>
<para>
Print a trace message if <literal>pred</literal> is false.
</para>
<para>
Intended to be used to augment asserts with helpful error messages.
</para>
<variablelist>
<varlistentry>
<term>
<varname>pred</varname>
</term>
<listitem>
<para>
Condition under which the <varname>msg</varname> should
<emphasis>not</emphasis> be printed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>msg</varname>
</term>
<listitem>
<para>
Message to print.
</para>
</listitem>
</varlistentry>
</variablelist>
<example xml:id="function-library-lib.asserts.assertMsg-example-false">
<title>Printing when the predicate is false</title>
<programlisting><![CDATA[
assert lib.asserts.assertMsg ("foo" == "bar") "foo is not bar, silly"
stderr> trace: foo is not bar, silly
stderr> assert failed
]]></programlisting>
</example>
</section>
<section xml:id="function-library-lib.asserts.assertOneOf">
<title><function>lib.asserts.assertOneOf</function></title>
<subtitle><literal>assertOneOf :: String -> String ->
StringList -> Bool</literal>
</subtitle>
<para>
Specialized <function>asserts.assertMsg</function> for checking if
<varname>val</varname> is one of the elements of <varname>xs</varname>.
Useful for checking enums.
</para>
<variablelist>
<varlistentry>
<term>
<varname>name</varname>
</term>
<listitem>
<para>
The name of the variable the user entered <varname>val</varname> into,
for inclusion in the error message.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>val</varname>
</term>
<listitem>
<para>
The value of what the user provided, to be compared against the values in
<varname>xs</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>xs</varname>
</term>
<listitem>
<para>
The list of valid values.
</para>
</listitem>
</varlistentry>
</variablelist>
<example xml:id="function-library-lib.asserts.assertOneOf-example">
<title>Ensuring a user provided a possible value</title>
<programlisting><![CDATA[
let sslLibrary = "bearssl";
in lib.asserts.assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ];
=> false
stderr> trace: sslLibrary must be one of "openssl", "libressl", but is: "bearssl"
]]></programlisting>
</example>
</section>
</section>
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