forked from mirrors/nixpkgs
f835f77e02
Covers assert functions and about half of the attrsets functions. Some internal consistency around IDs could be improved.
114 lines
2.9 KiB
XML
114 lines
2.9 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xml:id="sec-functions-library-asserts">
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<title>Assert functions</title>
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<section xml:id="function-library-lib.asserts.assertMsg">
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<title><function>lib.asserts.assertMsg</function></title>
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<subtitle><literal>assertMsg :: Bool -> String -> Bool</literal>
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</subtitle>
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<para>
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Print a trace message if <literal>pred</literal> is false.
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</para>
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<para>
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Intended to be used to augment asserts with helpful error messages.
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<varname>pred</varname>
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</term>
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<listitem>
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<para>
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Condition under which the <varname>msg</varname> should
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<emphasis>not</emphasis> be printed.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>msg</varname>
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</term>
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<listitem>
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<para>
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Message to print.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<example xml:id="function-library-lib.asserts.assertMsg-example-false">
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<title>Printing when the predicate is false</title>
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<programlisting><![CDATA[
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assert lib.asserts.assertMsg ("foo" == "bar") "foo is not bar, silly"
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stderr> trace: foo is not bar, silly
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stderr> assert failed
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]]></programlisting>
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</example>
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</section>
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<section xml:id="function-library-lib.asserts.assertOneOf">
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<title><function>lib.asserts.assertOneOf</function></title>
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<subtitle><literal>assertOneOf :: String -> String ->
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StringList -> Bool</literal>
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</subtitle>
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<para>
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Specialized <function>asserts.assertMsg</function> for checking if
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<varname>val</varname> is one of the elements of <varname>xs</varname>.
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Useful for checking enums.
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<varname>name</varname>
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</term>
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<listitem>
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<para>
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The name of the variable the user entered <varname>val</varname> into,
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for inclusion in the error message.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>val</varname>
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</term>
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<listitem>
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<para>
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The value of what the user provided, to be compared against the values in
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<varname>xs</varname>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>xs</varname>
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</term>
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<listitem>
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<para>
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The list of valid values.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<example xml:id="function-library-lib.asserts.assertOneOf-example">
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<title>Ensuring a user provided a possible value</title>
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<programlisting><![CDATA[
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let sslLibrary = "bearssl";
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in lib.asserts.assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ];
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=> false
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stderr> trace: sslLibrary must be one of "openssl", "libressl", but is: "bearssl"
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]]></programlisting>
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</example>
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</section>
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</section>
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