Mark Buildfile functions as pure or impure

17 files changed, 385 insertions, 299 deletions

diff --git a/libbuild2/bin/functions.cxx b/libbuild2/bin/functions.cxx
index 59fcdf2..1c6c0f4 100644
--- a/libbuild2/bin/functions.cxx
+++ b/libbuild2/bin/functions.cxx
@@ -24,12 +24,14 @@ namespace build2
       // bin.lib value on. As a result, it can be omitted in which case the
       // function call scope is used (covers project-local lib{} targets).
       //
+      // Note that this function is not pure.
+      //
       // @@ TODO: support for target (note that if it's out of project, then
       //          it's imported, which means it might still be qualified.)
       //
       // @@ TODO: support utility libraries (see link_member()).
       //
-      f[".link_member"] = [] (const scope* bs, names ns)
+      f.insert (".link_member", false) += [] (const scope* bs, names ns)
       {
         string t (convert<string> (move (ns)));
 
diff --git a/libbuild2/cc/functions.cxx b/libbuild2/cc/functions.cxx
index 98f7f97..c8b9d09 100644
--- a/libbuild2/cc/functions.cxx
+++ b/libbuild2/cc/functions.cxx
@@ -150,8 +150,10 @@ namespace build2
       // after all the specified library targets have been matched. Normally
       // it is used in ad hoc recipes to implement custom compilation.
       //
+      // Note that this function is not pure.
       //
-      f[".lib_poptions"].insert<lib_data, names, names> (
+      f.insert (".lib_poptions", false).
+        insert<lib_data, names, names> (
         &lib_thunk<appended_libraries>,
         lib_data {
           x,
@@ -189,8 +191,10 @@ namespace build2
       // after all the specified library targets have been matched. Normally
       // it is used in ad hoc recipes to implement custom linking.
       //
-      f[".lib_libs"].insert<lib_data,
-                            names, names, optional<names>, optional<names>> (
+      // Note that this function is not pure.
+      //
+      f.insert (".lib_libs", false).
+        insert<lib_data, names, names, optional<names>, optional<names>> (
         &lib_thunk<appended_libraries>,
         lib_data {
           x,
@@ -237,13 +241,15 @@ namespace build2
       //
       // Note that passing multiple targets at once is not a mere convenience:
       // this also allows for more effective duplicate suppression.
-
+      //
       // Note also that this function can only be called during execution
       // after all the specified library targets have been matched. Normally
       // it is used in ad hoc recipes to implement custom linking.
       //
-      f[".lib_rpaths"].insert<lib_data,
-                              names, names, optional<names>, optional<names>> (
+      // Note that this function is not pure.
+      //
+      f.insert (".lib_rpaths", false).
+        insert<lib_data, names, names, optional<names>, optional<names>> (
         &lib_thunk<rpathed_libraries>,
         lib_data {
           x,
diff --git a/libbuild2/config/functions.cxx b/libbuild2/config/functions.cxx
index aaf74ec..5e85238 100644
--- a/libbuild2/config/functions.cxx
+++ b/libbuild2/config/functions.cxx
@@ -27,7 +27,9 @@ namespace build2
       // config module creation was requested for other meta-operations with
       // config.config.module=true in bootstrap.build.
       //
-      f[".save"] = [] (const scope* s)
+      // Note that this function is not pure.
+      //
+      f.insert (".save", false) += [] (const scope* s)
       {
         if (s == nullptr)
           fail << "config.save() called out of scope" << endf;
diff --git a/libbuild2/function.cxx b/libbuild2/function.cxx
index 25dacf9..eaf3f9e 100644
--- a/libbuild2/function.cxx
+++ b/libbuild2/function.cxx
@@ -80,21 +80,6 @@ namespace build2
     return i != map_.end () && i->first.compare (0, n, name) == 0;
   }
 
-  auto function_map::
-  insert (string name, function_overload f) -> iterator
-  {
-    // Sanity checks.
-    //
-    assert (f.arg_min <= f.arg_max &&
-            f.arg_types.size () <= f.arg_max &&
-            f.impl != nullptr);
-
-    auto i (map_.emplace (move (name), move (f)));
-
-    i->second.name = i->first.c_str ();
-    return i;
-  }
-
   pair<value, bool> function_map::
   call (const scope* base,
         const string& name,
@@ -120,16 +105,17 @@ namespace build2
     // See the overall function machinery description for the ranking
     // semantics.
     //
-    auto ip (map_.equal_range (name));
+    const function_overloads* all_ovls (find (name));
 
     size_t rank (~0);
     small_vector<const function_overload*, 2> ovls;
+    if (all_ovls != nullptr)
     {
       size_t count (args.size ());
 
-      for (auto it (ip.first); it != ip.second; ++it)
+      for (auto it (all_ovls->begin ()); it != all_ovls->end (); ++it)
       {
-        const function_overload& f (it->second);
+        const function_overload& f (*it);
 
         // Argument count match.
         //
@@ -257,8 +243,11 @@ namespace build2
 
         dr << fail (loc) << "unmatched call to "; print_call (dr.os);
 
-        for (auto i (ip.first); i != ip.second; ++i)
-          dr << info << "candidate: " << i->second;
+        if (all_ovls != nullptr)
+        {
+          for (auto i (all_ovls->begin ()); i != all_ovls->end (); ++i)
+            dr << info << "candidate: " << *i;
+        }
 
         // If this is an unqualified name, then also print qualified
         // functions that end with this name. But skip functions that we
@@ -271,14 +260,20 @@ namespace build2
           for (auto i (begin ()); i != end (); ++i)
           {
             const string& q (i->first);
-            const function_overload& f (i->second);
 
-            if ((f.alt_name == nullptr || f.alt_name != name) &&
-                q.size () > n)
+            if (q.size () > n)
             {
-              size_t p (q.size () - n);
-              if (q[p - 1] == '.' && q.compare (p, n, name) == 0)
-                dr << info << "candidate: " << i->second;
+              for (auto j (i->second.begin ()); j != i->second.end (); ++j)
+              {
+                const function_overload& f (*j);
+
+                if (f.alt_name == nullptr || f.alt_name != name)
+                {
+                  size_t p (q.size () - n);
+                  if (q[p - 1] == '.' && q.compare (p, n, name) == 0)
+                    dr << info << "candidate: " << f;
+                }
+              }
             }
           }
         }
@@ -300,30 +295,8 @@ namespace build2
     }
   }
 
-  value function_family::
-  default_thunk (const scope* base,
-                 vector_view<value> args,
-                 const function_overload& f)
-  {
-    // Call the cast thunk.
-    //
-    struct cast_data // Prefix of function_cast::data.
-    {
-      value (*const thunk) (const scope*, vector_view<value>, const void*);
-    };
-
-    auto d (reinterpret_cast<const cast_data*> (&f.data));
-    return d->thunk (base, move (args), d);
-  }
-
-#if !defined(_WIN32)
-  constexpr const optional<const value_type*>* function_args<>::types;
-#else
-  const optional<const value_type*>* const function_args<>::types = nullptr;
-#endif
-
-  void function_family::entry::
-  insert (string n, function_overload f) const
+  auto function_family::
+  insert (string n, bool pure) const -> entry
   {
     // Figure out qualification.
     //
@@ -332,31 +305,47 @@ namespace build2
 
     if (p == string::npos)
     {
-      if (!qual.empty ())
+      if (!qual_.empty ())
       {
-        qn = qual;
+        qn = qual_;
         qn += '.';
         qn += n;
       }
     }
     else if (p == 0)
     {
-      assert (!qual.empty ());
-      n.insert (0, qual);
+      assert (!qual_.empty ());
+      n.insert (0, qual_);
     }
 
-    auto i (qn.empty () ? map_.end () : map_.insert (move (qn), f));
-    auto j (map_.insert (move (n), move (f)));
+    return entry {
+      map_.insert (move (n), pure),
+      qn.empty () ? nullptr : &map_.insert (move (qn), pure),
+      thunk_};
+  }
 
-    // If we have both, then set alternative names.
+  value function_family::
+  default_thunk (const scope* base,
+                 vector_view<value> args,
+                 const function_overload& f)
+  {
+    // Call the cast thunk.
     //
-    if (i != map_.end ())
+    struct cast_data // Prefix of function_cast::data.
     {
-      i->second.alt_name = j->first.c_str ();
-      j->second.alt_name = i->first.c_str ();
-    }
+      value (*const thunk) (const scope*, vector_view<value>, const void*);
+    };
+
+    auto d (reinterpret_cast<const cast_data*> (&f.data));
+    return d->thunk (base, move (args), d);
   }
 
+#if !defined(_WIN32)
+  constexpr const optional<const value_type*>* function_args<>::types;
+#else
+  const optional<const value_type*>* const function_args<>::types = nullptr;
+#endif
+
   // Static-initialize the function map and populate with builtin functions.
   //
 
diff --git a/libbuild2/function.hxx b/libbuild2/function.hxx
index 43b8024..8fdf8f4 100644
--- a/libbuild2/function.hxx
+++ b/libbuild2/function.hxx
@@ -75,14 +75,15 @@ namespace build2
   //
   // // Register length() and string.length().
   // //
-  // f["length"] = &string::size;
+  // f["length"] += &string::size;
   //
   // // Register string.max_size().
   // //
-  // f[".max_size"] = []() {return string ().max_size ();};
+  // f[".max_size"] += []() {return string ().max_size ();};
   //
-  // For more examples/ideas, study the existing function families (reside
-  // in the functions-*.cxx files).
+  // The use of += instead of = is meant to suggest that we are adding an
+  // overload. For more examples/ideas, study the existing function families
+  // (reside in the functions-*.cxx files).
   //
   // Note that normally there will be a function overload that has all the
   // parameters untyped with an implementation that falls back to one of the
@@ -90,6 +91,12 @@ namespace build2
   // from the argument value "syntax" (e.g., presence of a trailing slash for
   // a directory path).
   //
+  // A function is pure if for the same set of arguments it always produces
+  // the same result and has no (observable) side effects. Those functions
+  // that are not pure should be explicitly marked as such, for example:
+  //
+  // f.insert ("date", false /* pure */) += &date;
+  //
   struct function_overload;
 
   using function_impl = value (const scope*,
@@ -116,13 +123,13 @@ namespace build2
 
     using types = vector_view<const optional<const value_type*>>;
 
-    const size_t arg_min;
-    const size_t arg_max;
-    const types  arg_types;
+    size_t arg_min;
+    size_t arg_max;
+    types  arg_types;
 
     // Function implementation.
     //
-    function_impl* const impl;
+    function_impl* impl;
 
     // Auxiliary data storage. Note that it is expected to be trivially
     // copyable and destructible.
@@ -166,18 +173,57 @@ namespace build2
   LIBBUILD2_SYMEXPORT ostream&
   operator<< (ostream&, const function_overload&); // Print signature.
 
+  struct function_overloads: small_vector<function_overload, 8>
+  {
+    const char* name; // Set to point to key by function_map::insert() below.
+    bool pure = true;
+
+    function_overload&
+    insert (function_overload f)
+    {
+      // Sanity checks.
+      //
+      assert (f.arg_min <= f.arg_max           &&
+              f.arg_types.size () <= f.arg_max &&
+              f.impl != nullptr);
+
+      push_back (move (f));
+      back ().name = name;
+      return back ();
+    }
+  };
+
   class LIBBUILD2_SYMEXPORT function_map
   {
   public:
-    using map_type = std::multimap<string, function_overload>;
+    using map_type = std::map<string, function_overloads>;
     using iterator = map_type::iterator;
     using const_iterator = map_type::const_iterator;
 
-    iterator
-    insert (string name, function_overload);
+    function_overloads&
+    insert (string name, bool pure)
+    {
+      auto p (map_.emplace (move (name), function_overloads ()));
 
-    void
-    erase (iterator i) {map_.erase (i);}
+      function_overloads& r (p.first->second);
+
+      if (p.second)
+      {
+        r.name = p.first->first.c_str ();
+        r.pure = pure;
+      }
+      else
+        assert (r.pure == pure);
+
+      return r;
+    }
+
+    const function_overloads*
+    find (const string& name) const
+    {
+      auto i (map_.find (name));
+      return i != map_.end () ? &i->second : nullptr;
+    }
 
     value
     call (const scope* base,
@@ -263,6 +309,9 @@ namespace build2
     entry
     operator[] (string name) const;
 
+    entry
+    insert (string name, bool pure = true) const;
+
     static bool
     defined (function_map& map, string qual)
     {
@@ -762,20 +811,18 @@ namespace build2
 
   struct LIBBUILD2_SYMEXPORT function_family::entry
   {
-    function_map& map_;
-    string name;
-    const string& qual;
-    function_impl* thunk;
+    function_overloads& overloads;
+    function_overloads* alt_overloads;
+    function_impl*      thunk;
 
     template <typename R, typename... A>
     void
-    operator= (R (*impl) (A...)) &&
+    operator+= (R (*impl) (A...)) const
     {
       using args = function_args<A...>;
       using cast = function_cast_func<R, A...>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -786,13 +833,12 @@ namespace build2
 
     template <typename R, typename... A>
     void
-    operator= (R (*impl) (const scope*, A...)) &&
+    operator+= (R (*impl) (const scope*, A...)) const
     {
       using args = function_args<A...>;
       using cast = function_cast_func<R, const scope*, A...>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -812,20 +858,19 @@ namespace build2
 #if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 6
     template <typename L>
     void
-    operator= (const L&) &&
+    operator+= (const L&) const
     {
-      move (*this).coerce_lambda (&L::operator());
+      this->coerce_lambda (&L::operator());
     }
 
     template <typename L, typename R, typename... A>
     void
-    coerce_lambda (R (L::*op) (A...) const) &&
+    coerce_lambda (R (L::*op) (A...) const) const
     {
       using args = function_args<A...>;
       using cast = function_cast_lamb<L, R, A...>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -836,13 +881,12 @@ namespace build2
 
     template <typename L, typename R, typename... A>
     void
-    coerce_lambda (R (L::*op) (const scope*, A...) const) &&
+    coerce_lambda (R (L::*op) (const scope*, A...) const) const
     {
       using args = function_args<A...>;
       using cast = function_cast_lamb<L, R, const scope*, A...>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -853,9 +897,9 @@ namespace build2
 #else
     template <typename L>
     void
-    operator= (const L& l) &&
+    operator+= (const L& l) const
     {
-      move (*this).operator= (decay_lambda (&L::operator(), l));
+      this->operator+= (decay_lambda (&L::operator(), l));
     }
 
     template <typename L, typename R, typename... A>
@@ -875,13 +919,12 @@ namespace build2
     //
     template <typename R, typename T>
     void
-    operator= (R (T::*mf) () const) &&
+    operator+= (R (T::*mf) () const) const
     {
       using args = function_args<T>;
       using cast = function_cast_memf<R, T>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -894,13 +937,12 @@ namespace build2
     //
     template <typename R, typename T>
     void
-    operator= (R T::*dm) &&
+    operator+= (R T::*dm) const
     {
       using args = function_args<T>;
       using cast = function_cast_memd<R, T>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -915,12 +957,11 @@ namespace build2
     //
     template <typename D, typename... A>
     void
-    insert (function_impl* i, D d) &&
+    insert (function_impl* i, D d) const
     {
       using args = function_args<A...>;
 
-      insert (move (name),
-              function_overload (
+      insert (function_overload (
                 nullptr,
                 args::min,
                 args::max,
@@ -931,13 +972,27 @@ namespace build2
 
   private:
     void
-    insert (string, function_overload) const;
+    insert (function_overload f) const
+    {
+      function_overload* f1 (alt_overloads != nullptr
+                             ? &alt_overloads->insert (f)
+                             : nullptr);
+      function_overload& f2 (overloads.insert (move (f)));
+
+      // If we have both, then set alternative names.
+      //
+      if (f1 != nullptr)
+      {
+        f1->alt_name = f2.name;
+        f2.alt_name = f1->name;
+      }
+    }
   };
 
   inline auto function_family::
   operator[] (string name) const -> entry
   {
-    return entry {map_, move (name), qual_, thunk_};
+    return insert (move (name));
   }
 }
 
diff --git a/libbuild2/function.test.cxx b/libbuild2/function.test.cxx
index 514ac1e..f8a2c16 100644
--- a/libbuild2/function.test.cxx
+++ b/libbuild2/function.test.cxx
@@ -52,38 +52,37 @@ namespace build2
 
     function_family f (functions, "dummy");
 
-    f["fail"]     = []()        {fail << "failed" << endf;};
-    f["fail_arg"] = [](names a) {return convert<uint64_t> (move (a[0]));};
+    f["fail"]     += []()        {fail << "failed" << endf;};
+    f["fail_arg"] += [](names a) {return convert<uint64_t> (move (a[0]));};
 
-    f["nullable"] = [](names* a)          {return a == nullptr;};
-    f["optional"] = [](optional<names> a) {return !a;};
+    f["nullable"] += [](names* a)          {return a == nullptr;};
+    f["optional"] += [](optional<names> a) {return !a;};
 
-    f["dummy0"] = []()         {return "abc";};
-    f["dummy1"] = [](string s) {return s;};
-    f["dummy2"] = [](uint64_t x, uint64_t y) {return x + y;};
+    f["dummy0"] += []()         {return "abc";};
+    f["dummy1"] += [](string s) {return s;};
+    f["dummy2"] += [](uint64_t x, uint64_t y) {return x + y;};
 
-    f["ambig"] = [](names a, optional<string>)   {return a;};
-    f["ambig"] = [](names a, optional<uint64_t>) {return a;};
+    f["ambig"] += [](names a, optional<string>)   {return a;};
+    f["ambig"] += [](names a, optional<uint64_t>) {return a;};
 
-    f["reverse"] = [](names a) {return a;};
+    f["reverse"] += [](names a) {return a;};
 
-    f["scoped"]      = [](const scope*, names a) {return a;};
-    f["scoped_void"] = [](const scope*, names) {};
-    f["scoped"]      = &scoped;
-    f["scoped_void"] = &scoped_void;
+    f["scoped"]      += [](const scope*, names a) {return a;};
+    f["scoped_void"] += [](const scope*, names) {};
+    f["scoped"]      += &scoped;
+    f["scoped_void"] += &scoped_void;
 
-    f[".qual"] = []() {return "abc";};
+    f[".qual"] += []() {return "abc";};
 
-    f[".length"] = &path::size; // Member function.
-    f[".type"]   = &name::type; // Data member.
+    f[".length"] += &path::size; // Member function.
+    f[".type"]   += &name::type; // Data member.
 
-    f[".abs"] = [](dir_path d) {return d.absolute ();};
+    f[".abs"] += [](dir_path d) {return d.absolute ();};
 
     // Variadic function with first required argument of type bool. Returns
     // number of arguments passed.
     //
-    functions.insert (
-      "variadic",
+    functions.insert ("variadic", true).insert (
       function_overload (
         nullptr,
         1,
@@ -96,8 +95,7 @@ namespace build2
 
     // Dump arguments.
     //
-    functions.insert (
-      "dump",
+    functions.insert ("dump", true).insert (
       function_overload (
         nullptr,
         0,
diff --git a/libbuild2/functions-builtin.cxx b/libbuild2/functions-builtin.cxx
index 97cc449..4689ac2 100644
--- a/libbuild2/functions-builtin.cxx
+++ b/libbuild2/functions-builtin.cxx
@@ -19,7 +19,9 @@ namespace build2
     // Note that we may want to extend the scope argument to a more general
     // notion of "lookup context" (scope, target, prerequisite).
     //
-    f["defined"] = [](const scope* s, names name)
+    // Note that this function is not pure.
+    //
+    f.insert ("defined", false) += [](const scope* s, names name)
     {
       if (s == nullptr)
         fail << "defined() called out of scope" << endf;
@@ -29,7 +31,9 @@ namespace build2
 
     // Return variable visibility if it has been entered and NULL otherwise.
     //
-    f["visibility"] = [](const scope* s, names name)
+    // Note that this function is not pure.
+    //
+    f.insert ("visibility", false) += [](const scope* s, names name)
     {
       if (s == nullptr)
         fail << "visibility() called out of scope" << endf;
@@ -42,25 +46,25 @@ namespace build2
               : nullopt);
     };
 
-    f["type"] = [](value* v) {return v->type != nullptr ? v->type->name : "";};
-    f["null"]  = [](value* v) {return v->null;};
-    f["empty"] = [](value* v)  {return v->null || v->empty ();};
+    f["type"] += [](value* v) {return v->type != nullptr ? v->type->name : "";};
+    f["null"] += [](value* v) {return v->null;};
+    f["empty"] += [](value* v)  {return v->null || v->empty ();};
 
-    f["identity"] = [](value* v) {return move (*v);};
+    f["identity"] += [](value* v) {return move (*v);};
 
     // string
     //
-    f["string"] = [](bool b) {return b ? "true" : "false";};
-    f["string"] = [](int64_t i) {return to_string (i);};
-    f["string"] = [](uint64_t i) {return to_string (i);};
-    f["string"] = [](name n) {return to_string (n);};
+    f["string"] += [](bool b) {return b ? "true" : "false";};
+    f["string"] += [](int64_t i) {return to_string (i);};
+    f["string"] += [](uint64_t i) {return to_string (i);};
+    f["string"] += [](name n) {return to_string (n);};
 
     // Quote a value returning its string representation. If escape is true,
     // then also escape (with a backslash) the quote characters being added
     // (this is useful if the result will be re-parsed, for example as a
     // Testscript command line).
     //
-    f["quote"] = [](value* v, optional<value> escape)
+    f["quote"] += [](value* v, optional<value> escape)
     {
       if (v->null)
         return string ();
@@ -81,7 +85,9 @@ namespace build2
     // Return NULL if the environment variable is not set, untyped value
     // otherwise.
     //
-    f["getenv"] = [](names name)
+    // Note that this function is not pure.
+    //
+    f.insert ("getenv", false) += [](names name)
     {
       optional<string> v (getenv (convert<string> (move (name))));
 
diff --git a/libbuild2/functions-filesystem.cxx b/libbuild2/functions-filesystem.cxx
index 54c5315..696b1cf 100644
--- a/libbuild2/functions-filesystem.cxx
+++ b/libbuild2/functions-filesystem.cxx
@@ -78,6 +78,9 @@ namespace build2
   void
   filesystem_functions (function_map& m)
   {
+    // @@ Maybe we should have the ability to mark the whole family as not
+    //    pure?
+
     function_family f (m, "filesystem");
 
     // path_search
@@ -86,25 +89,32 @@ namespace build2
     // absolute path, then the start directory is ignored (if present).
     // Otherwise, the start directory must be specified and be absolute.
     //
-    f["path_search"] = [](path pattern, optional<dir_path> start)
+    // Note that this function is not pure.
+    //
     {
-      return path_search (pattern, start);
-    };
+      auto e (f.insert ("path_search", false));
 
-    f["path_search"] = [](path pattern, names start)
-    {
-      return path_search (pattern, convert<dir_path> (move (start)));
-    };
+      e += [](path pattern, optional<dir_path> start)
+      {
+        return path_search (pattern, start);
+      };
 
-    f["path_search"] = [](names pattern, optional<dir_path> start)
-    {
-      return path_search (convert<path> (move (pattern)), start);
-    };
+      e += [](path pattern, names start)
+      {
+        return path_search (pattern, convert<dir_path> (move (start)));
+      };
+
+      e += [](names pattern, optional<dir_path> start)
+      {
+        return path_search (convert<path> (move (pattern)), start);
+      };
+
+      e += [](names pattern, names start)
+      {
+        return path_search (convert<path>     (move (pattern)),
+                            convert<dir_path> (move (start)));
+      };
+    }
 
-    f["path_search"] = [](names pattern, names start)
-    {
-      return path_search (convert<path>     (move (pattern)),
-                          convert<dir_path> (move (start)));
-    };
   }
 }
diff --git a/libbuild2/functions-name.cxx b/libbuild2/functions-name.cxx
index 8b61c81..e821e52 100644
--- a/libbuild2/functions-name.cxx
+++ b/libbuild2/functions-name.cxx
@@ -59,11 +59,11 @@ namespace build2
     //
     function_family fn (m, "name");
 
-    fn["name"] = [](const scope* s, name n)
+    fn["name"] += [](const scope* s, name n)
     {
       return to_target_name (s, move (n)).first.value;
     };
-    fn["name"] = [](const scope* s, names ns)
+    fn["name"] += [](const scope* s, names ns)
     {
       return to_target_name (s, convert<name> (move (ns))).first.value;
     };
@@ -71,40 +71,40 @@ namespace build2
     // Note: returns NULL if extension is unspecified (default) and empty if
     // specified as no extension.
     //
-    fn["extension"] = [](const scope* s, name n)
+    fn["extension"] += [](const scope* s, name n)
     {
       return to_target_name (s, move (n)).second;
     };
-    fn["extension"] = [](const scope* s, names ns)
+    fn["extension"] += [](const scope* s, names ns)
     {
       return to_target_name (s, convert<name> (move (ns))).second;
     };
 
-    fn["directory"] = [](const scope* s, name n)
+    fn["directory"] += [](const scope* s, name n)
     {
       return to_target_name (s, move (n)).first.dir;
     };
-    fn["directory"] = [](const scope* s, names ns)
+    fn["directory"] += [](const scope* s, names ns)
     {
       return to_target_name (s, convert<name> (move (ns))).first.dir;
     };
 
-    fn["target_type"] = [](const scope* s, name n)
+    fn["target_type"] += [](const scope* s, name n)
     {
       return to_target_name (s, move (n)).first.type;
     };
-    fn["target_type"] = [](const scope* s, names ns)
+    fn["target_type"] += [](const scope* s, names ns)
     {
       return to_target_name (s, convert<name> (move (ns))).first.type;
     };
 
     // Note: returns NULL if no project specified.
     //
-    fn["project"] = [](const scope* s, name n)
+    fn["project"] += [](const scope* s, name n)
     {
       return to_target_name (s, move (n)).first.proj;
     };
-    fn["project"] = [](const scope* s, names ns)
+    fn["project"] += [](const scope* s, names ns)
     {
       return to_target_name (s, convert<name> (move (ns))).first.proj;
     };
@@ -113,7 +113,12 @@ namespace build2
     //
     function_family ft (m, "target");
 
-    ft["path"] = [](const scope* s, names ns)
+    // Note that while this function is not technically pure, we don't mark it
+    // as such since it can only be called (normally form a recipe) after the
+    // target has been matched, meaning that this target is a prerequisite and
+    // therefore this impurity has been accounted for.
+    //
+    ft["path"] += [](const scope* s, names ns)
     {
       if (s == nullptr)
         fail << "target.path() called out of scope";
@@ -155,7 +160,10 @@ namespace build2
     // This one can only be called on a single target since we don't support
     // containers of process_path's (though we probably could).
     //
-    fn["process_path"] = [](const scope* s, names ns)
+    // Note that while this function is not technically pure, we don't mark it
+    // as such for the same reasons as $path() above.
+    //
+    fn["process_path"] += [](const scope* s, names ns)
     {
       if (s == nullptr)
         fail << "target.process_path() called out of scope";
@@ -184,7 +192,7 @@ namespace build2
     //
     function_family fb (m, "builtin");
 
-    fb[".concat"] = [](dir_path d, name n)
+    fb[".concat"] += [](dir_path d, name n)
     {
       d /= n.dir;
       n.dir = move (d);
diff --git a/libbuild2/functions-path.cxx b/libbuild2/functions-path.cxx
index 2d5fad4..fdf712c 100644
--- a/libbuild2/functions-path.cxx
+++ b/libbuild2/functions-path.cxx
@@ -144,9 +144,9 @@ namespace build2
 
     // string
     //
-    f["string"] = [](path p) {return move (p).string ();};
+    f["string"] += [](path p) {return move (p).string ();};
 
-    f["string"] = [](paths v)
+    f["string"] += [](paths v)
     {
       strings r;
       for (auto& p: v)
@@ -154,7 +154,7 @@ namespace build2
       return r;
     };
 
-    f["string"] = [](dir_paths v)
+    f["string"] += [](dir_paths v)
     {
       strings r;
       for (auto& p: v)
@@ -164,9 +164,9 @@ namespace build2
 
     // representation
     //
-    f["representation"] = [](path p) {return move (p).representation ();};
+    f["representation"] += [](path p) {return move (p).representation ();};
 
-    f["representation"] = [](paths v)
+    f["representation"] += [](paths v)
     {
       strings r;
       for (auto& p: v)
@@ -174,7 +174,7 @@ namespace build2
       return r;
     };
 
-    f["representation"] = [](dir_paths v)
+    f["representation"] += [](dir_paths v)
     {
       strings r;
       for (auto& p: v)
@@ -186,24 +186,24 @@ namespace build2
     //
     // @@ TODO: add ability to specify alternative separator.
     //
-    f["canonicalize"] = [](path p)     {p.canonicalize (); return p;};
-    f["canonicalize"] = [](dir_path p) {p.canonicalize (); return p;};
+    f["canonicalize"] += [](path p)     {p.canonicalize (); return p;};
+    f["canonicalize"] += [](dir_path p) {p.canonicalize (); return p;};
 
-    f["canonicalize"] = [](paths v)
+    f["canonicalize"] += [](paths v)
     {
       for (auto& p: v)
         p.canonicalize ();
       return v;
     };
 
-    f["canonicalize"] = [](dir_paths v)
+    f["canonicalize"] += [](dir_paths v)
     {
       for (auto& p: v)
         p.canonicalize ();
       return v;
     };
 
-    f[".canonicalize"] = [](names ns)
+    f[".canonicalize"] += [](names ns)
     {
       // For each path decide based on the presence of a trailing slash
       // whether it is a directory. Return as untyped list of (potentially
@@ -221,19 +221,22 @@ namespace build2
 
     // normalize
     //
-    f["normalize"] = [](path p, optional<value> a)
+    // @@ TODO: normalize(true) is not pure, redo as a separate actualize()
+    //    function.
+    //
+    f["normalize"] += [](path p, optional<value> a)
     {
       p.normalize (a && convert<bool> (move (*a)));
       return p;
     };
 
-    f["normalize"] = [](dir_path p, optional<value> a)
+    f["normalize"] += [](dir_path p, optional<value> a)
     {
       p.normalize (a && convert<bool> (move (*a)));
       return p;
     };
 
-    f["normalize"] = [](paths v, optional<value> a)
+    f["normalize"] += [](paths v, optional<value> a)
     {
       bool act (a && convert<bool> (move (*a)));
 
@@ -242,7 +245,7 @@ namespace build2
 
       return v;
     };
-    f["normalize"] = [](dir_paths v, optional<value> a)
+    f["normalize"] += [](dir_paths v, optional<value> a)
     {
       bool act (a && convert<bool> (move (*a)));
 
@@ -251,7 +254,7 @@ namespace build2
       return v;
     };
 
-    f[".normalize"] = [](names ns, optional<value> a)
+    f[".normalize"] += [](names ns, optional<value> a)
     {
       bool act (a && convert<bool> (move (*a)));
 
@@ -271,9 +274,9 @@ namespace build2
 
     // directory
     //
-    f["directory"] = &path::directory;
+    f["directory"] += &path::directory;
 
-    f["directory"] = [](paths v)
+    f["directory"] += [](paths v)
     {
       dir_paths r;
       for (const path& p: v)
@@ -281,14 +284,14 @@ namespace build2
       return r;
     };
 
-    f["directory"] = [](dir_paths v)
+    f["directory"] += [](dir_paths v)
     {
       for (dir_path& p: v)
         p = p.directory ();
       return v;
     };
 
-    f[".directory"] = [](names ns)
+    f[".directory"] += [](names ns)
     {
       // For each path decide based on the presence of a trailing slash
       // whether it is a directory. Return as list of directory names.
@@ -305,23 +308,23 @@ namespace build2
 
     // base
     //
-    f["base"] = &path::base;
+    f["base"] += &path::base;
 
-    f["base"] = [](paths v)
+    f["base"] += [](paths v)
     {
       for (path& p: v)
         p = p.base ();
       return v;
     };
 
-    f["base"] = [](dir_paths v)
+    f["base"] += [](dir_paths v)
     {
       for (dir_path& p: v)
         p = p.base ();
       return v;
     };
 
-    f[".base"] = [](names ns)
+    f[".base"] += [](names ns)
     {
       // For each path decide based on the presence of a trailing slash
       // whether it is a directory. Return as untyped list of (potentially
@@ -339,28 +342,28 @@ namespace build2
 
     // leaf
     //
-    f["leaf"] = &path::leaf;
+    f["leaf"] += &path::leaf;
 
-    f["leaf"] = [](path p, dir_path d)
+    f["leaf"] += [](path p, dir_path d)
     {
       return leaf (p, move (d));
     };
 
-    f["leaf"] = [](paths v, optional<dir_path> d)
+    f["leaf"] += [](paths v, optional<dir_path> d)
     {
       for (path& p: v)
         p = leaf (p, d);
       return v;
     };
 
-    f["leaf"] = [](dir_paths v, optional<dir_path> d)
+    f["leaf"] += [](dir_paths v, optional<dir_path> d)
     {
       for (dir_path& p: v)
         p = leaf (p, d);
       return v;
     };
 
-    f[".leaf"] = [](names ns, optional<dir_path> d)
+    f[".leaf"] += [](names ns, optional<dir_path> d)
     {
       // For each path decide based on the presence of a trailing slash
       // whether it is a directory. Return as untyped list of (potentially
@@ -378,9 +381,9 @@ namespace build2
 
     // extension
     //
-    f["extension"] = &extension;
+    f["extension"] += &extension;
 
-    f[".extension"] = [](names ns)
+    f[".extension"] += [](names ns)
     {
       return extension (convert<path> (move (ns)));
     };
@@ -415,19 +418,19 @@ namespace build2
     //
     // Name matching.
     //
-    f[".match"] = [](string name, string pattern)
+    f[".match"] += [](string name, string pattern)
     {
       return path_match (name, pattern);
     };
 
     // Path matching.
     //
-    f["match"] = [](path ent, path pat, optional<dir_path> start)
+    f["match"] += [](path ent, path pat, optional<dir_path> start)
     {
       return path_match (ent, pat, start);
     };
 
-    f["match"] = [](path ent, names pat, optional<names> start)
+    f["match"] += [](path ent, names pat, optional<names> start)
     {
       return path_match (ent,
                          convert<path> (move (pat)),
@@ -436,7 +439,7 @@ namespace build2
                          : optional<dir_path> ());
     };
 
-    f["match"] = [](names ent, path pat, optional<names> start)
+    f["match"] += [](names ent, path pat, optional<names> start)
     {
       return path_match (convert<path> (move (ent)),
                          pat,
@@ -448,7 +451,7 @@ namespace build2
     // The semantics depends on the presence of the start directory or the
     // first two argument syntactic representation.
     //
-    f[".match"] = [](names ent, names pat, optional<names> start)
+    f[".match"] += [](names ent, names pat, optional<names> start)
     {
       auto path_arg = [] (const names& a) -> bool
       {
@@ -472,15 +475,15 @@ namespace build2
     //
     function_family b (m, "builtin", &path_thunk);
 
-    b[".concat"] = &concat_path_string;
-    b[".concat"] = &concat_dir_path_string;
+    b[".concat"] += &concat_path_string;
+    b[".concat"] += &concat_dir_path_string;
 
-    b[".concat"] = [](path l, names ur)
+    b[".concat"] += [](path l, names ur)
     {
       return concat_path_string (move (l), convert<string> (move (ur)));
     };
 
-    b[".concat"] = [](dir_path l, names ur)
+    b[".concat"] += [](dir_path l, names ur)
     {
       return concat_dir_path_string (move (l), convert<string> (move (ur)));
     };
diff --git a/libbuild2/functions-process-path.cxx b/libbuild2/functions-process-path.cxx
index 86d4445..199ad0d 100644
--- a/libbuild2/functions-process-path.cxx
+++ b/libbuild2/functions-process-path.cxx
@@ -16,8 +16,8 @@ namespace build2
 
       // As discussed in value_traits<process_path>, we always have recall.
       //
-      f["recall"] = &process_path::recall;
-      f["effect"] = [](process_path p)
+      f["recall"] += &process_path::recall;
+      f["effect"] += [](process_path p)
       {
         return move (p.effect.empty () ? p.recall : p.effect);
       };
@@ -26,8 +26,8 @@ namespace build2
     {
       function_family f (m, "process_path_ex");
 
-      f["name"] = &process_path_ex::name;
-      f["checksum"] = &process_path_ex::checksum;
+      f["name"] += &process_path_ex::name;
+      f["checksum"] += &process_path_ex::checksum;
     }
   }
 }
diff --git a/libbuild2/functions-process.cxx b/libbuild2/functions-process.cxx
index 73ca916..0870874 100644
--- a/libbuild2/functions-process.cxx
+++ b/libbuild2/functions-process.cxx
@@ -414,12 +414,14 @@ namespace build2
     //
     // Run builtin or external program and return trimmed stdout.
     //
-    f[".run"] = [](const scope* s, names args)
+    // Note that this function is not pure.
+    //
+    f.insert (".run", false) += [](const scope* s, names args)
     {
       return run (s, move (args));
     };
 
-    f["run"] = [](const scope* s, process_path pp)
+    f.insert ("run", false) += [](const scope* s, process_path pp)
     {
       return run_process (s, pp, strings ());
     };
@@ -433,38 +435,41 @@ namespace build2
     // (as a whole) against <pat> and, if successful, returned, optionally
     // processed with <fmt>, as an element of a list.
     //
-    f[".run_regex"] = [](const scope* s, names a, string p, optional<string> f)
+    // Note that this function is not pure.
+    //
     {
-      return run_regex (s, move (a), p, f);
-    };
+      auto e (f.insert (".run_regex", false));
 
-    f[".run_regex"] = [] (const scope* s, names a, names p, optional<names> f)
-    {
-      return run_regex (s,
-                        move (a),
-                        convert<string> (move (p)),
-                        f ? convert<string> (move (*f)) : nullopt_string);
-    };
+      e += [](const scope* s, names a, string p, optional<string> f)
+      {
+        return run_regex (s, move (a), p, f);
+      };
 
-    f["run_regex"] = [](const scope* s,
-                        process_path pp,
-                        string p,
-                        optional<string> f)
+      e += [] (const scope* s, names a, names p, optional<names> f)
+      {
+        return run_regex (s,
+                          move (a),
+                          convert<string> (move (p)),
+                          f ? convert<string> (move (*f)) : nullopt_string);
+      };
+    }
     {
-      return run_process_regex (s, pp, strings (), p, f);
-    };
+      auto e (f.insert ("run_regex", false));
 
-    f["run_regex"] = [](const scope* s,
-                        process_path pp,
-                        names p,
-                        optional<names> f)
-    {
-      return run_process_regex (s,
-                                pp, strings (),
-                                convert<string> (move (p)),
-                                (f
-                                 ? convert<string> (move (*f))
-                                 : nullopt_string));
-    };
+      e += [](const scope* s, process_path pp, string p, optional<string> f)
+      {
+        return run_process_regex (s, pp, strings (), p, f);
+      };
+
+      e += [](const scope* s, process_path pp, names p, optional<names> f)
+      {
+        return run_process_regex (s,
+                                  pp, strings (),
+                                  convert<string> (move (p)),
+                                  (f
+                                   ? convert<string> (move (*f))
+                                   : nullopt_string));
+      };
+    }
   }
 }
diff --git a/libbuild2/functions-project-name.cxx b/libbuild2/functions-project-name.cxx
index 86206b0..145e62c 100644
--- a/libbuild2/functions-project-name.cxx
+++ b/libbuild2/functions-project-name.cxx
@@ -13,46 +13,46 @@ namespace build2
   {
     function_family f (m, "project_name");
 
-    f["string"] = [](project_name p) {return move (p).string ();};
+    f["string"] += [](project_name p) {return move (p).string ();};
 
-    f["base"] = [](project_name p, optional<string> ext)
+    f["base"] += [](project_name p, optional<string> ext)
     {
       return ext ? p.base (ext->c_str ()) : p.base ();
     };
 
-    f["base"] = [](project_name p, names ext)
+    f["base"] += [](project_name p, names ext)
     {
       return p.base (convert<string> (move (ext)).c_str ());
     };
 
-    f["extension"] = &project_name::extension;
-    f["variable"]  = &project_name::variable;
+    f["extension"] += &project_name::extension;
+    f["variable"]  += &project_name::variable;
 
     // Project name-specific overloads from builtins.
     //
     function_family b (m, "builtin");
 
-    b[".concat"] = [](project_name n, string s)
+    b[".concat"] += [](project_name n, string s)
     {
       string r (move (n).string ());
       r += s;
       return r;
     };
 
-    b[".concat"] = [](string s, project_name n)
+    b[".concat"] += [](string s, project_name n)
     {
       s += n.string ();
       return s;
     };
 
-    b[".concat"] = [](project_name n, names ns)
+    b[".concat"] += [](project_name n, names ns)
     {
       string r (move (n).string ());
       r += convert<string> (move (ns));
       return r;
     };
 
-    b[".concat"] = [](names ns, project_name n)
+    b[".concat"] += [](names ns, project_name n)
     {
       string r (convert<string> (move (ns)));
       r += n.string ();
diff --git a/libbuild2/functions-regex.cxx b/libbuild2/functions-regex.cxx
index a59d808..46d71ba 100644
--- a/libbuild2/functions-regex.cxx
+++ b/libbuild2/functions-regex.cxx
@@ -521,12 +521,12 @@ namespace build2
     //               sub-strings that match the marked sub-expressions and
     //               NULL if no match
     //
-    f[".match"] = [](value s, string re, optional<names> flags)
+    f[".match"] += [](value s, string re, optional<names> flags)
     {
       return match (move (s), re, move (flags));
     };
 
-    f[".match"] = [](value s, names re, optional<names> flags)
+    f[".match"] += [](value s, names re, optional<names> flags)
     {
       return match (move (s), convert<string> (move (re)), move (flags));
     };
@@ -540,12 +540,12 @@ namespace build2
     //
     // icase - match ignoring case
     //
-    f[".find_match"] = [](names s, string re, optional<names> flags)
+    f[".find_match"] += [](names s, string re, optional<names> flags)
     {
       return find_match (move (s), re, move (flags));
     };
 
-    f[".find_match"] = [](names s, names re, optional<names> flags)
+    f[".find_match"] += [](names s, names re, optional<names> flags)
     {
       return find_match (move (s), convert<string> (move (re)), move (flags));
     };
@@ -573,12 +573,12 @@ namespace build2
     // If both return_match and return_subs flags are specified then the
     // sub-string that matches the whole regular expression comes first.
     //
-    f[".search"] = [](value s, string re, optional<names> flags)
+    f[".search"] += [](value s, string re, optional<names> flags)
     {
       return search (move (s), re, move (flags));
     };
 
-    f[".search"] = [](value s, names re, optional<names> flags)
+    f[".search"] += [](value s, names re, optional<names> flags)
     {
       return search (move (s), convert<string> (move (re)), move (flags));
     };
@@ -593,12 +593,12 @@ namespace build2
     //
     // icase - match ignoring case
     //
-    f[".find_search"] = [](names s, string re, optional<names> flags)
+    f[".find_search"] += [](names s, string re, optional<names> flags)
     {
       return find_search (move (s), re, move (flags));
     };
 
-    f[".find_search"] = [](names s, names re, optional<names> flags)
+    f[".find_search"] += [](names s, names re, optional<names> flags)
     {
       return find_search (move (s),
                           convert<string> (move (re)),
@@ -625,12 +625,12 @@ namespace build2
     // If both format_first_only and format_no_copy flags are specified then
     // the result will only contain the replacement of the first match.
     //
-    f[".replace"] = [](value s, string re, string fmt, optional<names> flags)
+    f[".replace"] += [](value s, string re, string fmt, optional<names> flags)
     {
       return replace (move (s), re, fmt, move (flags));
     };
 
-    f[".replace"] = [](value s, names re, names fmt, optional<names> flags)
+    f[".replace"] += [](value s, names re, names fmt, optional<names> flags)
     {
       return replace (move (s),
                       convert<string> (move (re)),
@@ -658,7 +658,7 @@ namespace build2
     // Note that if format_no_copy is specified, unmatched lines are not
     // copied either.
     //
-    f[".replace_lines"] = [](value s,
+    f[".replace_lines"] += [](value s,
                              string re,
                              string fmt,
                              optional<names> flags)
@@ -666,7 +666,7 @@ namespace build2
       return replace_lines (move (s), re, move (fmt), move (flags));
     };
 
-    f[".replace_lines"] = [](value s,
+    f[".replace_lines"] += [](value s,
                              names re,
                              names* fmt,
                              optional<names> flags)
@@ -695,12 +695,12 @@ namespace build2
     //
     // format_no_copy    - do not copy unmatched value parts into the result
     //
-    f[".split"] = [](value s, string re, string fmt, optional<names> flags)
+    f[".split"] += [](value s, string re, string fmt, optional<names> flags)
     {
       return split (move (s), re, fmt, move (flags));
     };
 
-    f[".split"] = [](value s, names re, names fmt, optional<names> flags)
+    f[".split"] += [](value s, names re, names fmt, optional<names> flags)
     {
       return split (move (s),
                     convert<string> (move (re)),
@@ -730,7 +730,7 @@ namespace build2
     // the result will be a concatenation of only the first match
     // replacements.
     //
-    f[".merge"] = [](names s,
+    f[".merge"] += [](names s,
                      string re,
                      string fmt,
                      optional<string> delim,
@@ -739,7 +739,7 @@ namespace build2
       return merge (move (s), re, fmt, move (delim), move (flags));
     };
 
-    f[".merge"] = [](names s,
+    f[".merge"] += [](names s,
                      names re,
                      names fmt,
                      optional<names> delim,
@@ -775,12 +775,12 @@ namespace build2
     // the result elements will only contain the replacement of the first
     // match.
     //
-    f[".apply"] = [](names s, string re, string fmt, optional<names> flags)
+    f[".apply"] += [](names s, string re, string fmt, optional<names> flags)
     {
       return apply (move (s), re, fmt, move (flags));
     };
 
-    f[".apply"] = [](names s, names re, names fmt, optional<names> flags)
+    f[".apply"] += [](names s, names re, names fmt, optional<names> flags)
     {
       return apply (move (s),
                     convert<string> (move (re)),
diff --git a/libbuild2/functions-string.cxx b/libbuild2/functions-string.cxx
index c5cb216..b430ebf 100644
--- a/libbuild2/functions-string.cxx
+++ b/libbuild2/functions-string.cxx
@@ -13,31 +13,31 @@ namespace build2
   {
     function_family f (m, "string");
 
-    f["string"] = [](string s)  {return s;};
+    f["string"] += [](string s)  {return s;};
 
     // @@ Shouldn't it concatenate elements into the single string?
     // @@ Doesn't seem to be used so far. Can consider removing.
     //
-    // f["string"] = [](strings v) {return v;};
+    // f["string"] += [](strings v) {return v;};
 
     // Compare ASCII strings ignoring case and returning the boolean value.
     //
-    f["icasecmp"] = [](string x, string y)
+    f["icasecmp"] += [](string x, string y)
     {
       return icasecmp (x, y) == 0;
     };
 
-    f["icasecmp"] = [](string x, names y)
+    f["icasecmp"] += [](string x, names y)
     {
       return icasecmp (x, convert<string> (move (y))) == 0;
     };
 
-    f["icasecmp"] = [](names x, string y)
+    f["icasecmp"] += [](names x, string y)
     {
       return icasecmp (convert<string> (move (x)), y) == 0;
     };
 
-    f[".icasecmp"] = [](names x, names y)
+    f[".icasecmp"] += [](names x, names y)
     {
       return icasecmp (convert<string> (move (x)),
                        convert<string> (move (y))) == 0;
@@ -45,34 +45,34 @@ namespace build2
 
     // Trim.
     //
-    f["trim"] = [](string s)
+    f["trim"] += [](string s)
     {
       return trim (move (s));
     };
 
-    f[".trim"] = [](names s)
+    f[".trim"] += [](names s)
     {
       return names {name (trim (convert<string> (move (s))))};
     };
 
     // Convert ASCII strings into lower/upper case.
     //
-    f["lcase"] = [](string s)
+    f["lcase"] += [](string s)
     {
       return lcase (move (s));
     };
 
-    f[".lcase"] = [](names s)
+    f[".lcase"] += [](names s)
     {
       return names {name (lcase (convert<string> (move (s))))};
     };
 
-    f["ucase"] = [](string s)
+    f["ucase"] += [](string s)
     {
       return ucase (move (s));
     };
 
-    f[".ucase"] = [](names s)
+    f[".ucase"] += [](names s)
     {
       return names {name (ucase (convert<string> (move (s))))};
     };
@@ -81,15 +81,15 @@ namespace build2
     //
     function_family b (m, "builtin");
 
-    b[".concat"] = [](string l, string r) {l += r; return l;};
+    b[".concat"] += [](string l, string r) {l += r; return l;};
 
-    b[".concat"] = [](string l, names ur)
+    b[".concat"] += [](string l, names ur)
     {
       l += convert<string> (move (ur));
       return l;
     };
 
-    b[".concat"] = [](names ul, string r)
+    b[".concat"] += [](names ul, string r)
     {
       string l (convert<string> (move (ul)));
       l += r;
diff --git a/libbuild2/functions-target-triplet.cxx b/libbuild2/functions-target-triplet.cxx
index ff9a15d..4b0ec02 100644
--- a/libbuild2/functions-target-triplet.cxx
+++ b/libbuild2/functions-target-triplet.cxx
@@ -13,22 +13,22 @@ namespace build2
   {
     function_family f (m, "target_triplet");
 
-    f["string"] = [](target_triplet t) {return t.string ();};
-    f["representation"] = [](target_triplet t) {return t.representation ();};
+    f["string"] += [](target_triplet t) {return t.string ();};
+    f["representation"] += [](target_triplet t) {return t.representation ();};
 
     // Target triplet-specific overloads from builtins.
     //
     function_family b (m, "builtin");
 
-    b[".concat"] = [](target_triplet l, string sr) {return l.string () + sr;};
-    b[".concat"] = [](string sl, target_triplet r) {return sl + r.string ();};
+    b[".concat"] += [](target_triplet l, string sr) {return l.string () + sr;};
+    b[".concat"] += [](string sl, target_triplet r) {return sl + r.string ();};
 
-    b[".concat"] = [](target_triplet l, names ur)
+    b[".concat"] += [](target_triplet l, names ur)
     {
       return l.string () + convert<string> (move (ur));
     };
 
-    b[".concat"] = [](names ul, target_triplet r)
+    b[".concat"] += [](names ul, target_triplet r)
     {
       return convert<string> (move (ul)) + r.string ();
     };
diff --git a/libbuild2/install/functions.cxx b/libbuild2/install/functions.cxx
index c780061..5668efe 100644
--- a/libbuild2/install/functions.cxx
+++ b/libbuild2/install/functions.cxx
@@ -18,7 +18,9 @@ namespace build2
       // Resolve potentially relative install.* value to an absolute directory
       // based on (other) install.* values visible from the calling scope.
       //
-      f[".resolve"] = [] (const scope* s, dir_path d)
+      // Note that this function is not pure.
+      //
+      f.insert (".resolve", false) += [] (const scope* s, dir_path d)
       {
         if (s == nullptr)
           fail << "install.resolve() called out of scope" << endf;