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authorEelco Dolstra <eelco.dolstra@logicblox.com>2016-08-17T13·12+0200
committerEelco Dolstra <eelco.dolstra@logicblox.com>2016-08-17T15·19+0200
commit22d6e31fc6a9de2ee424984e629ccd2e394ba512 (patch)
tree0c8ea6b2799583edf3597f7723883ba4c1655c7e /src/libexpr/primops.cc
parentac841a46797a91eaddb3e0ad193c505fc49da597 (diff)
Add a mechanism for derivation attributes to reference the derivation's outputs
For example, you can now say:

  configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";

The strings returned by the ‘placeholder’ builtin are replaced at
build time by the actual store paths corresponding to the specified
outputs.

Previously, you had to work around the inability to self-reference by doing stuff like:

  preConfigure = ''
    configureFlags+=" --prefix $out --includedir=$dev"
  '';

or rely on ad-hoc variable interpolation semantics in Autoconf or Make
(e.g. --prefix=\$(out)), which doesn't always work.
Diffstat (limited to 'src/libexpr/primops.cc')
-rw-r--r--src/libexpr/primops.cc14
1 files changed, 14 insertions, 0 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 6a4a7a0356fd..51ee16353ef6 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -673,6 +673,19 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 }
 
 
+/* Return a placeholder string for the specified output that will be
+   substituted by the corresponding output path at build time. For
+   example, ‘placeholder "out"’ returns the string
+   /1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9. At build
+   time, any occurence of this string in an derivation attribute will
+   be replaced with the concrete path in the Nix store of the output
+   ‘out’. */
+static void prim_placeholder(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    mkString(v, hashPlaceholder(state.forceStringNoCtx(*args[0], pos)));
+}
+
+
 /*************************************************************
  * Paths
  *************************************************************/
@@ -1893,6 +1906,7 @@ void EvalState::createBaseEnv()
 
     // Derivations
     addPrimOp("derivationStrict", 1, prim_derivationStrict);
+    addPrimOp("placeholder", 1, prim_placeholder);
 
     // Networking
     addPrimOp("__fetchurl", 1, prim_fetchurl);