From 4a053bfdfd85915a2a659a337bd171bc22c49138 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Wed, 23 Aug 2006 14:39:11 +0000
Subject: [PATCH] * A new primop `builtins', which returns an attribute set
 containing   all the primops.  This allows Nix expressions to test for new  
 primops and take appropriate action if they're not available.  For  
 instance, rather than calling a primop `foo' directly, they could   say `if
 builtins ? foo then builtins.foo ... else ...'.

---
 src/libexpr/primops.cc            | 26 ++++++++++++++++++++++++++
 tests/lang/eval-okay-builtins.exp |  1 +
 tests/lang/eval-okay-builtins.nix | 12 ++++++++++++
 3 files changed, 39 insertions(+)
 create mode 100644 tests/lang/eval-okay-builtins.exp
 create mode 100644 tests/lang/eval-okay-builtins.nix

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index dfc565b43..bc4db2d81 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -7,6 +7,30 @@
 #include "nixexpr-ast.hh"
 
 
+static Expr primBuiltins(EvalState & state, const ATermVector & args)
+{
+    /* Return an attribute set containing all primops.  This allows
+       Nix expressions to test for new primops and take appropriate
+       action if they're not available.  For instance, rather than
+       calling a primop `foo' directly, they could say `if builtins ?
+       foo then builtins.foo ... else ...'. */
+
+    ATermMap builtins(128);
+
+    for (ATermMap::const_iterator i = state.primOps.begin();
+         i != state.primOps.end(); ++i)
+    {
+        string name = aterm2String(i->key);
+        if (string(name, 0, 2) == "__")
+            name = string(name, 2);
+        /* !!! should use makePrimOp here, I guess. */
+        builtins.set(toATerm(name), makeAttrRHS(makeVar(i->key), makeNoPos()));
+    }
+
+    return makeAttrs(builtins);
+}
+
+
 /* Load and evaluate an expression from path specified by the
    argument. */ 
 static Expr primImport(EvalState & state, const ATermVector & args)
@@ -660,6 +684,8 @@ static Expr primRelativise(EvalState & state, const ATermVector & args)
 
 void EvalState::addPrimOps()
 {
+    addPrimOp("builtins", 0, primBuiltins);
+        
     addPrimOp("true", 0, primTrue);
     addPrimOp("false", 0, primFalse);
     addPrimOp("null", 0, primNull);
diff --git a/tests/lang/eval-okay-builtins.exp b/tests/lang/eval-okay-builtins.exp
new file mode 100644
index 000000000..f4f3ba81a
--- /dev/null
+++ b/tests/lang/eval-okay-builtins.exp
@@ -0,0 +1 @@
+Path("/foo")
diff --git a/tests/lang/eval-okay-builtins.nix b/tests/lang/eval-okay-builtins.nix
new file mode 100644
index 000000000..e9d65e88a
--- /dev/null
+++ b/tests/lang/eval-okay-builtins.nix
@@ -0,0 +1,12 @@
+assert builtins ? currentSystem;
+assert !builtins ? __currentSystem;
+
+let {
+
+  x = if builtins ? dirOf then builtins.dirOf /foo/bar else "";
+
+  y = if builtins ? fnord then builtins.fnord "foo" else "";
+
+  body = x + y;
+  
+}