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doc: Illustrate procedures that return packages.

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* doc/guix.texi (Defining Package Variants): Illustrate procedures that
return packages.
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Ludovic Courtès 2020-11-03 11:01:37 +01:00
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@ -6908,11 +6908,44 @@ The @code{alist-delete} call above removes the tuple from the
(@pxref{SRFI-1 Association Lists,,, guile, GNU Guile Reference
Manual}).
In some cases, you may find it useful to write functions
(``procedures'', in Scheme parlance) that return a package based on some
parameters. For example, consider the @code{luasocket} library for the
Lua programming language. We want to create @code{luasocket} packages
for major versions of Lua. One way to do that is to define a procedure
that takes a Lua package and returns a @code{luasocket} package that
depends on it:
@lisp
(define (make-lua-socket name lua)
;; Return a luasocket package built with LUA.
(package
(name name)
(version "3.0")
;; several fields omitted
(inputs
`(("lua" ,lua)))
(synopsis "Socket library for Lua")))
(define-public lua5.1-socket
(make-lua-socket "lua5.1-socket" lua-5.1))
(define-public lua5.2-socket
(make-lua-socket "lua5.2-socket" lua-5.2))
@end lisp
Here we have defined packages @code{lua5.1-socket} and
@code{lua5.2-socket} by calling @code{make-lua-socket} with different
arguments. @xref{Procedures,,, guile, GNU Guile Reference Manual}, for
more info on procedures. Having top-level public definitions for these
two packages means that they can be referred to from the command line
(@pxref{Package Modules}).
@cindex package transformations
These are pretty simple package variants. As a convenience, the
@code{(guix transformations)} module provides a high-level interface
that directly maps to package transformation options (@pxref{Package
Transformation Options}):
that directly maps to the more sophisticated package transformation
options (@pxref{Package Transformation Options}):
@deffn {Scheme Procedure} options->transformation @var{opts}
Return a procedure that, when passed an object to build (package,