From 4e043c2f32af3d3d49c53a2d06746e2fd6967836 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 31 Mar 2022 16:01:50 +0200
Subject: [PATCH] Document isPure()

---
 src/libstore/derivations.hh | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index b62e40786..98e59b64e 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -135,7 +135,10 @@ struct DerivationType : _DerivationTypeRaw {
        separately. Never true for non-CA derivations. */
     bool needsNetworkAccess() const;
 
-    /* FIXME */
+    /* Whether the derivation is expected to produce the same result
+       every time, and therefore it only needs to be built once. This
+       is only false for derivations that have the attribute '__impure
+       = true'. */
     bool isPure() const;
 
     /* Does the derivation knows its own output paths?