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doc: Update gexp wrt. extensibility. 

This is a followup to bcb1328.

* doc/guix.texi (G-Expressions): Move paragraph about extensibility
  below.  Remove assumptions that things are either packages or
  derivations.
master
Ludovic Courtès 2015-03-28 21:16:36 +01:00
parent 065b795497
commit b39fc6f7bc
1 changed files with 16 additions and 15 deletions
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@ -2490,13 +2490,9 @@ Gexps are meant to be written to a file and run or manipulated by other
processes.
@item
When a package or derivation is unquoted inside a gexp, the result is as
if its output file name had been introduced.
Actually this mechanism is not limited to package and derivation
objects; @dfn{compilers} able to ``lower'' other high-level objects to
derivations can be defined, such that these objects can also be inserted
into gexps.
When a high-level object such as a package or derivation is unquoted
inside a gexp, the result is as if its output file name had been
introduced.
@item
Gexps carry information about the packages or derivations they refer to,
@ -2504,6 +2500,11 @@ and these dependencies are automatically added as inputs to the build
processes that use them.
@end itemize
Actually this mechanism is not limited to package and derivation
objects; @dfn{compilers} able to ``lower'' other high-level objects to
derivations can be defined, such that these objects can also be inserted
into gexps.
To illustrate the idea, here is an example of a gexp:
@example
@ -2563,24 +2564,24 @@ or more of the following forms:
@table @code
@item #$@var{obj}
@itemx (ungexp @var{obj})
Introduce a reference to @var{obj}. @var{obj} may be a package or a
Introduce a reference to @var{obj}. @var{obj} may have one of the
supported types, for example a package or a
derivation, in which case the @code{ungexp} form is replaced by its
output file name---e.g., @code{"/gnu/store/@dots{}-coreutils-8.22}.
If @var{obj} is a list, it is traversed and any package or derivation
references are substituted similarly.
If @var{obj} is a list, it is traversed and references to supported
objects are substituted similarly.
If @var{obj} is another gexp, its contents are inserted and its
dependencies are added to those of the containing gexp.
If @var{obj} is another kind of object, it is inserted as is.
@item #$@var{package-or-derivation}:@var{output}
@itemx (ungexp @var{package-or-derivation} @var{output})
@item #$@var{obj}:@var{output}
@itemx (ungexp @var{obj} @var{output})
This is like the form above, but referring explicitly to the
@var{output} of @var{package-or-derivation}---this is useful when
@var{package-or-derivation} produces multiple outputs (@pxref{Packages
with Multiple Outputs}).
@var{output} of @var{obj}---this is useful when @var{obj} produces
multiple outputs (@pxref{Packages with Multiple Outputs}).
@item #+@var{obj}
@itemx #+@var{obj}:output