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doc: Cross chapter: Add note on why use example platforms

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John Ericson 2018-01-26 12:30:05 -05:00
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doc/cross-compilation.xml
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@ -256,6 +256,16 @@
You can use them programmatically, or on the command line: <programlisting>
nix-build &lt;nixpkgs&gt; --arg crossSystem '(import &lt;nixpkgs/lib&gt;).systems.examples.fooBarBaz' -A whatever</programlisting>
</para>
<note>
<para>
Eventually we would like to make these platform examples an unnecessary convenience so that <programlisting>
nix-build &lt;nixpkgs&gt; --arg crossSystem.config '&lt;arch&gt;-&lt;os&gt;-&lt;vendor&gt;-&lt;abi&gt;' -A whatever</programlisting>
works in the vast majority of cases.
The problem today is dependencies on other sorts of configuration which aren't given proper defaults.
We rely on the examples to crudely to set those configuration parameters in some vaguely sane manner on the users behalf.
Issue <link xlink:href="https://github.com/NixOS/nixpkgs/issues/34274">#34274</link> tracks this inconvenience along with its root cause in crufty configuration options.
</para>
</note>
<para>
While one is free to pass both parameters in full, there's a lot of logic to fill in missing fields.
As discussed in the previous section, only one of <varname>system</varname>, <varname>config</varname>, and <varname>parsed</varname> is needed to infer the other two.