291 lines
9.9 KiB
XML
291 lines
9.9 KiB
XML
<chapter>
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<title>Introduction</title>
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<sect1>
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<title>The problem space</title>
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<para>
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Nix is a system for controlling the automatic creation and distribution
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of data, such as computer programs and other software artifacts. This is
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a very general problem, and there are many applications that fall under
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this description.
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</para>
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<sect2>
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<title>Build management</title>
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<para>
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Build management tools are used to perform <emphasis>software
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builds</emphasis>, that is, the construction of derived products
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(<emphasis>derivates)</emphasis>) such as executable programs from
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source code. A commonly used build tool is Make, which is a standard
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tool on Unix systems. These tools have to deal with several issues:
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>Efficiency</emphasis>. Since building large systems
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can take a substantial amount of time, it is desirable that build
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steps that have been performed in the past are not repeated
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unnecessarily, i.e., if a new build differs from a previous build
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only with respect to certain sources, then only the build steps
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that (directly or indirectly) <emphasis>depend</emphasis> on
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those sources should be redone.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Correctness</emphasis> is this context means that the
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derivates produced by a build are always consistent with the
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sources, that is, they are equal to what we would get if we were
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to build the derivates from those sources. This requirement is
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trivially met when we do a full, unconditional build, but is far
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from trivial under the requirement of efficiency, since it is not
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easy to determine which derivates are affected by a change to a
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source.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Variability</emphasis> is the property that a software
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system can be built in a (potentially large) number of variants.
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Variation exists both in <emphasis>time</emphasis>---the
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evolution of different versions of an artifact---and in
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<emphasis>space</emphasis>---the artifact might have
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configuration options that lead to variants that differ in the
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features they support (for example, a system might be built with
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or without debugging information).
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</para>
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<para>
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Build managers historically have had good support for variation
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in time (rebuilding the system in an intelligent way when sources
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change is one of the primary reasons to use a build manager), but
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not always for variation in space. For example,
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<command>make</command> will not automatically ensure that
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variant builds are properly isolated from each other (they will
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in fact overwrite each other unless special precautions are
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taken).
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>High-level system modelling language</emphasis>. The
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language in which one describes what and how derivates are to be
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produced should have sufficient abstraction facilities to make it
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easy to specify the derivation of even very large systems. Also,
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the language should be <emphasis>modular</emphasis> to enable
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components from possible different sources to be easily combined.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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<sect2>
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<title>Package management</title>
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<para>
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After software has been built, is must also be
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<emphasis>deployed</emphasis> in the intended target environment, e.g.,
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the user's workstation. Examples include the Red Hat package manager
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(RPM), Microsoft's MSI, and so on. Here also we have several issues to
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contend with:
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<itemizedlist>
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<listitem>
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<para>
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The <emphasis>creation</emphasis> of packages from some formal
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description of what artifacts should be distributed in the
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package.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>deployment</emphasis> of packages, that is, the
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mechanism by which we get them onto the intended target
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environment. This can be as simple as copying a file, but
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complexity comes from the wide range of possible installation
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media (such as a network install), and the scalability of the
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process (if a program must be installed on a thousand systems, we
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do not want to visit each system and perform some manual steps to
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install the program on that system; that is, the complexity for
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the system administrator should be constant, not linear).
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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</sect1>
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<!--######################################################################-->
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<sect1>
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<title>What Nix provides</title>
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<para>
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Here is a summary of Nix's main features:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>Reliable dependencies.</emphasis> Builds of file system
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objects depend on other file system object, such as source files,
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tools, and so on. We would like to ensure that a build does not
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refer to any objects that have not been declared as inputs for that
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build. This is important for several reasons. First, if any of the
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inputs change, we need to rebuild the things that depend on them to
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maintain consistency between sources and derivates. Second, when we
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<emphasis>deploy</emphasis> file system objects (that is, copy them
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to a different system), we want to be certain that we copy everything
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that we need.
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</para>
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<para>
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Nix ensures this by building and storing file system objects in paths
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that are infeasible to predict in advance. For example, the
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artifacts of a package <literal>X</literal> might be stored in
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<filename>/nix/store/d58a0606ed616820de291d594602665d-X</filename>,
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rather than in, say, <filename>/usr/lib</filename>. The path
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component <filename>d58a...</filename> is actually a cryptographic
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hash of all the inputs (i.e., sources, requisites, and build flags)
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used in building <literal>X</literal>, and as such is very fragile:
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any change to the inputs will change the hash. Therefore it is not
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sensible to <emphasis>hard-code</emphasis> such a path into the build
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scripts of a package <literal>Y</literal> that uses
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<literal>X</literal> (as does happen with <quote>fixed</quote> paths
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such as <filename>/usr/lib</filename>). Rather, the build script of
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package <literal>Y</literal> is parameterised with the actual
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location of <literal>X</literal>, which is supplied by the Nix
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system.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Support for variability.</emphasis>
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</para>
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<para>
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As stated above, the path name of a file system object contain a
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cryptographic hash of all inputs involved in building it. A change to
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any of the inputs will cause the hash to change--and by extension,
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the path name. These inputs include both sources (variation in time)
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and configuration options (variation in space). Therefore variants
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of the same package don't clash---they can co-exist peacefully within
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the same file system. So thanks to Nix's mechanism for reliably
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dealing with dependencies, we obtain management of variants for free
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(or, to quote Simon Peyton-Jone, it's not free, but it has already
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been paid for).
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Transparent source/binary deployment.</emphasis>
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Easy configuration duplication.</emphasis>
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Automatic storage management.</emphasis>
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Atomic upgrades and rollbacks.</emphasis>
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Support for many simultaneous configurations.</emphasis>
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>Portability.</emphasis> Nix is quite portable. Contrary
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to build systems like those in, e.g., Vesta and ClearCase [sic?], it
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does not rely on operating system extensions.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Here is what Nix doesn't yet provide, but will:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>Build management.</emphasis> In principle it is already
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possible to do build management using Fix (by writing builders that
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perform appropriate build steps), but the Fix language is not yet
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powerful enough to make this pleasant. The <ulink
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url='http://www.cs.uu.nl/~eelco/maak/'>Maak build manager</ulink>
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should be retargeted to produce Nix expressions, or alternatively,
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extend Fix with Maak's semantics and concrete syntax (since Fix needs
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a concrete syntax anyway). Another interesting idea is to write a
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<command>make</command> implementation that uses Nix as a back-end to
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support <ulink
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url='http://www.research.att.com/~bs/bs_faq.html#legacy'>legacy</ulink>
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build files.
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</para>
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</listitem>
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</itemizedlist>
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</sect1>
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<!--######################################################################-->
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<sect1>
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<title>The Nix system</title>
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<para>
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...
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</para>
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<para>
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Existing tools in this field generally both a underlying model (such as
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the derivation graph of build tools, or the versioning scheme that
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determines when two packages are <quote>compatible</quote> in a package
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management system) and a formalism that allows ...
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</para>
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<para>
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Following the principle of separation of mechanism and policy, the Nix
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system separates the <emphasis>low-level aspect</emphasis> of file system
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object management form the <emphasis>high-level aspect</emphasis> of the
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...
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</para>
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</sect1>
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</chapter>
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end:
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