* Two primops: builtins.intersectAttrs and builtins.functionArgs.

  intersectAttrs returns the (right-biased) intersection between two
  attribute sets, e.g. every attribute from the second set that also
  exists in the first.  functionArgs returns the set of attributes
  expected by a function.

  The main goal of these is to allow the elimination of most of
  all-packages.nix.  Most package instantiations in all-packages.nix
  have this form:

    foo = import ./foo.nix {
      inherit a b c;
    };

  With intersectAttrs and functionArgs, this can be written as:

    foo = callPackage (import ./foo.nix) { };

  where

   callPackage = f: args:
     f ((builtins.intersectAttrs (builtins.functionArgs f) pkgs) // args);

  I.e., foo.nix is called with all attributes from "pkgs" that it
  actually needs (e.g., pkgs.a, pkgs.b and pkgs.c).  (callPackage can
  do any other generic package-level stuff we might want, such as
  applying makeOverridable.)  Of course, the automatically supplied
  arguments can be overriden if needed, e.g.

    foo = callPackage (import ./foo.nix) {
      c = c_version_2;
    };

  but for the vast majority of packages, this won't be needed.

  The advantages are to reduce the amount of typing needed to add a
  dependency (from three sites to two), and to reduce the number of
  trivial commits to all-packages.nix.  For the former, there have
  been two previous attempts:

    - Use "args: with args;" in the package's function definition.
      This however obscures the actual expected arguments of a
      function, which is very bad.

    - Use "{ arg1, arg2, ... }:" in the package's function definition
      (i.e. use the ellipis "..." to allow arbitrary additional
      arguments), and then call the function with all of "pkgs" as an
      argument.  But this inhibits error detection if you call it with
      an misspelled (or obsolete) argument.

1 files changed, 66 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 783d26c44818..6408ca9569c5 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -812,6 +812,70 @@ static Expr prim_isAttrs(EvalState & state, const ATermVector & args)
 }
 
 
+/* Return the right-biased intersection of two attribute sets as1 and
+   as2, i.e. a set that contains every attribute from as2 that is also
+   a member of as1. */
+static Expr prim_intersectAttrs(EvalState & state, const ATermVector & args)
+{
+    ATermMap as1, as2;
+    queryAllAttrs(evalExpr(state, args[0]), as1, true);
+    queryAllAttrs(evalExpr(state, args[1]), as2, true);
+
+    ATermMap res;
+    foreach (ATermMap::const_iterator, i, as2)
+        if (as1[i->key]) res.set(i->key, i->value);
+
+    return makeAttrs(res);
+}
+
+
+static void attrsInPattern(ATermMap & map, Pattern pat)
+{
+    ATerm name;
+    ATermList formals;
+    Pattern pat1, pat2;
+    ATermBool ellipsis;
+    if (matchAttrsPat(pat, formals, ellipsis)) { 
+        for (ATermIterator i(formals); i; ++i) {
+            ATerm def;
+            if (!matchFormal(*i, name, def)) abort();
+            map.set(name, makeAttrRHS(makeBool(def != constNoDefaultValue), makeNoPos()));
+        }
+    }
+    else if (matchAtPat(pat, pat1, pat2)) {
+        attrsInPattern(map, pat1);
+        attrsInPattern(map, pat2);
+    }
+}
+
+
+/* Return a set containing the names of the formal arguments expected
+   by the function `f'.  The value of each attribute is a Boolean
+   denoting whether has a default value.  For instance,
+
+      functionArgs ({ x, y ? 123}: ...)
+   => { x = false; y = true; }
+
+   "Formal argument" here refers to the attributes pattern-matched by
+   the function.  Plain lambdas are not included, e.g.
+
+      functionArgs (x: ...)
+   => { }
+*/
+static Expr prim_functionArgs(EvalState & state, const ATermVector & args)
+{
+    Expr f = evalExpr(state, args[0]);
+    ATerm pat, body, pos;
+    if (!matchFunction(f, pat, body, pos))
+        throw TypeError("`functionArgs' required a function");
+    
+    ATermMap as;
+    attrsInPattern(as, pat);
+
+    return makeAttrs(as);
+}
+
+
 /*************************************************************
  * Lists
  *************************************************************/
@@ -1070,6 +1134,8 @@ void EvalState::addPrimOps()
     addPrimOp("__isAttrs", 1, prim_isAttrs);
     addPrimOp("removeAttrs", 2, prim_removeAttrs);
     addPrimOp("__listToAttrs", 1, prim_listToAttrs);
+    addPrimOp("__intersectAttrs", 2, prim_intersectAttrs);
+    addPrimOp("__functionArgs", 1, prim_functionArgs);
 
     // Lists
     addPrimOp("__isList", 1, prim_isList);