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doc: Explain "file-like objects". 

* doc/guix.texi (G-Expressions): Mention "file-like objects" and explain
  more.
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Ludovic Courtès 2015-06-05 14:53:32 +02:00
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@ -2942,12 +2942,12 @@ and these dependencies are automatically added as inputs to the build
processes that use them. processes that use them.
@end itemize @end itemize
Actually this mechanism is not limited to package and derivation This mechanism is not limited to package and derivation
objects; @dfn{compilers} able to ``lower'' other high-level objects to objects: @dfn{compilers} able to ``lower'' other high-level objects to
derivations can be defined, such that these objects can also be inserted derivations can be defined, such that these objects can also be inserted
into gexps. Another useful type of high-level object that can be into gexps. For example, a useful type of high-level object that can be
inserted in a gexp is @dfn{local files}, which allows files from the inserted in a gexp is ``file-like objects'', which make it easy to
local file system to be added to the store and referred to by add files to the store and refer to them in
derivations and such (see @code{local-file} and @code{plain-file} derivations and such (see @code{local-file} and @code{plain-file}
below.) below.)
@ -3113,6 +3113,24 @@ refer to. Any reference to another store item will lead to a build error.
The other arguments are as for @code{derivation} (@pxref{Derivations}). The other arguments are as for @code{derivation} (@pxref{Derivations}).
@end deffn @end deffn
@cindex file-like objects
The @code{local-file} and @code{plain-file} procedures below return
@dfn{file-like objects}. That is, when unquoted in a G-expression,
these objects lead to a file in the store. Consider this G-expression:
@example
#~(system* (string-append #$glibc "/sbin/nscd") "-f"
#$(local-file "/tmp/my-nscd.conf"))
@end example
The effect here is to ``intern'' @file{/tmp/my-nscd.conf} by copying it
to the store. Once expanded, for instance @i{via}
@code{gexp->derivation}, the G-expression refers to that copy under
@file{/gnu/store}; thus, modifying or removing the file in @file{/tmp}
does not have any effect on what the G-expression does.
@code{plain-file} can be used similarly; it differs in that the file
content is directly passed as a string.
@deffn {Scheme Procedure} local-file @var{file} [@var{name}] @ @deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
[#:recursive? #t] [#:recursive? #t]
Return an object representing local file @var{file} to add to the store; this Return an object representing local file @var{file} to add to the store; this