06/09: derivations: Improve docstring of 'substitution-oracle'.

[Ludovic Courtès]

civodul pushed a commit to branch master
in repository guix.

commit 1eabf4ec63587a90fd8ea05a07f832d240dc3967
Author: Ludovic Courtès <address@hidden>
Date:   Wed Jul 22 18:35:43 2015 +0200

    derivations: Improve docstring of 'substitution-oracle'.
    
    * guix/derivations.scm (substitution-oracle): Improve docstring.
---
 guix/derivations.scm |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/guix/derivations.scm b/guix/derivations.scm
index d694ccf..342a6c8 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -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