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derivations: Improve docstring of 'substitution-oracle'. 

* guix/derivations.scm (substitution-oracle): Improve docstring.
This commit is contained in:
Ludovic Courtès 2015-07-22 18:35:43 +02:00
parent b59df2434a
commit 1eabf4ec63
1 changed files with 4 additions and 2 deletions
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guix/derivations.scm
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@ -242,8 +242,10 @@ result is the set of prerequisites of DRV not already in valid."
(define* (substitution-oracle store drv)
"Return a one-argument procedure that, when passed a store file name,
returns #t if it's substitutable and #f otherwise. The returned procedure
knows about all substitutes for all the derivations listed in DRV; it also
knows about their prerequisites, unless they are themselves substitutable.
knows about all substitutes for all the derivations listed in DRV, *except*
those that are already valid (that is, it won't bother checking whether an
item is substitutable if it's already on disk); it also knows about their
prerequisites, unless they are themselves substitutable.
Creating a single oracle (thus making a single 'substitutable-paths' call) and
reusing it is much more efficient than calling 'has-substitutes?' or similar