From 22d6e31fc6a9de2ee424984e629ccd2e394ba512 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Wed, 17 Aug 2016 15:12:54 +0200
Subject: Add a mechanism for derivation attributes to reference the
 derivation's outputs
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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.
---
 src/libexpr/primops.cc | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

(limited to 'src/libexpr/primops.cc')

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);
-- 
cgit 1.4.1