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* New language feature: with expressions.

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The expression `with E1; E2' evaluates to E2 with all bindings in
  the attribute set E1 substituted.  E.g.,

    with {x = 123;}; x

  evaluates to 123.  That is, the attribute set E1 is in scope in E2.

  This is particularly useful when importing files containing lots
  definitions.  E.g., instead of

    let {
      inherit (import ./foo.nix) a b c d e f;

      body = ... a ... f ...;
    }

  we can now say

    with import ./foo.nix;

    ... a ... f ...

  I.e., we don't have to say what variables should be brought into scope.
This commit is contained in:
Eelco Dolstra 2004-10-25 16:54:56 +00:00
parent f4d44a0026
commit 37d7abd694
7 changed files with 61 additions and 27 deletions
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34
doc/manual/package-management.xml
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@ -17,18 +17,12 @@ available for installation.</para>
<para>In Nix, different users can have different <quote>views</quote>
on the set of installed applications. That is, there might be lots of
applications present on the system (possibly in many different
versions), but users can have a specific selection of those
active — where <quote>active</quote> just means that it appears
in a directory in the user's <envar>PATH</envar>.</para>
<para>Such a view on the set of installed applications is called a
<emphasis>user environment</emphasis>, which is just a directory tree
consisting of symlinks to the files of the active applications. In
Nix, operations such as upgrading or removing components never
overwrite or remove the files of those components, and they don't even
touch the user environments that point to them. Rather, they cause a
<emphasis>new</emphasis> user environment to be constructed based on
the old one.</para>
versions), but users can have a specific selection of those active —
where <quote>active</quote> just means that it appears in a directory
in the user's <envar>PATH</envar>. Such a view on the set of
installed applications is called a <emphasis>user
environment</emphasis>, which is just a directory tree consisting of
symlinks to the files of the active applications. </para>
<para>Components are installed from a set of <emphasis>Nix
expressions</emphasis> that tell Nix how to build those components,
@ -168,7 +162,21 @@ set.</para></footnote></para>
<sect1><title>Profiles</title>
<para>Bla</para>
<para>In Nix, operations such as upgrading or removing components
never overwrite or remove the files of those components, and they
don't even touch the user environments that point to them. Rather,
they cause a <emphasis>new</emphasis> user environment to be
constructed based on the old one. This is illustrated in Figure
bla.</para>
<figure><title>User environments</title>
<mediaobject>
<imageobject>
<imagedata fileref='figures/userenv-frame3.png' format='PNG' />
</imageobject>
</mediaobject>
</figure>
</sect1>