doc: Document fixed-output derivations.

* doc/guix.texi (Derivations): Add paragraph on fixed-output
derivations.
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Ludovic Courtès 2017-12-21 22:12:00 +01:00
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@ -4210,6 +4210,14 @@ paths}. Derivations paths can be passed to the @code{build-derivations}
procedure to perform the build actions they prescribe (@pxref{The
Store}).
@cindex fixed-output derivations
Operations such as file downloads and version-control checkouts for
which the expected content hash is known in advance are modeled as
@dfn{fixed-output derivations}. Unlike regular derivations, the outputs
of a fixed-output derivation are independent of its inputs---e.g., a
source code download produces the same result regardless of the download
method and tools being used.
The @code{(guix derivations)} module provides a representation of
derivations as Scheme objects, along with procedures to create and
otherwise manipulate derivations. The lowest-level primitive to create