From bddc83a1487e9f3c1bb3ac2279c0238e8e6c3ff3 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Wed, 16 May 2007 16:17:04 +0000
Subject: [PATCH] * New builtin function "isFunction".  You're not supposed to
 use it   ;-) * Channels: fix channels that are plain lists of derivations
 (like   strategoxt-unstable) instead  of functions (like nixpkgs-unstable).  
 This fixes the error message "error: the left-hand side of the   function
 call is neither a function nor a primop (built-in   operation) but a list".

---
 corepkgs/channels/unpack.sh.in |  2 +-
 src/libexpr/eval.cc            |  2 +-
 src/libexpr/primops.cc         | 13 +++++++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/corepkgs/channels/unpack.sh.in b/corepkgs/channels/unpack.sh.in
index 03c6e5b2f..1f2886a6a 100644
--- a/corepkgs/channels/unpack.sh.in
+++ b/corepkgs/channels/unpack.sh.in
@@ -24,7 +24,7 @@ for ((n = 0; n < ${#inputs[*]}; n += 2)); do
     @coreutils@/mv * ../$dirName # !!! hacky
     
     attrName=$(echo $dirName | @tr@ -- '- ' '__')
-    echo "$attrName = import ./$dirName {};" >> $expr
+    echo "$attrName = let e = import ./$dirName; in if builtins.isFunction e then e {} else e;" >> $expr
 done
 
 echo '} // {_combineChannels = true;}' >> $expr
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 552b58625..a8a22e2f2 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -478,7 +478,7 @@ LocalNoInline(Expr evalCall(EvalState & state, Expr fun, Expr arg))
     }
 
     else throwTypeError(
-        "the left-hand side of the function call is neither a function nor a primop (built-in operation) but %1%",
+        "attempt to call something which is neither a function nor a primop (built-in operation) but %1%",
         showType(fun));
 }
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index c7fbca0bb..95440c9bf 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -114,6 +114,18 @@ static Expr prim_isNull(EvalState & state, const ATermVector & args)
 }
 
 
+/* Determine whether the argument is a function. */
+static Expr prim_isFunction(EvalState & state, const ATermVector & args)
+{
+    Expr e = evalExpr(state, args[0]);
+    ATermList formals;
+    ATerm name, body, pos;
+    return makeBool(
+        matchFunction(e, formals, body, pos) ||
+        matchFunction1(e, name, body, pos));
+}
+
+
 static Path findDependency(Path dir, string dep)
 {
     if (dep[0] == '/') throw EvalError(
@@ -884,6 +896,7 @@ void EvalState::addPrimOps()
     // Miscellaneous
     addPrimOp("import", 1, prim_import);
     addPrimOp("isNull", 1, prim_isNull);
+    addPrimOp("__isFunction", 1, prim_isFunction);
     addPrimOp("dependencyClosure", 1, prim_dependencyClosure);
     addPrimOp("abort", 1, prim_abort);
     addPrimOp("throw", 1, prim_throw);