Add string_map buildfile value type

This exposes the std::map<std::string,std::string> type to buildfiles.

New functions:

  $size(<string-map>)
  $keys(<string-map>)

Subscript can be used to lookup a value by key. The result is [null] if
there is no value associated with the specified key. For example:

map = [string_map] a@1 b@2 c@3

b = ($map[b])  # 2

if ($map[z] == [null])
  ...

Note that append (+=) is overriding (like std::map::insert_or_assign())
while prepend (=+) is not (like std::map::insert()). In a sense, whatever
appears last (from left to right) is kept, which is consistent with what
we expect to happen when specifying the same key repeatedly in a literal
representation. For example:

map = [string_map] a@0 b@2 a@1  # a@1 b@2
map += b@0 c@3                  # a@1 b@0 c@3
map =+ b@1 d@4                  # a@1 b@0 c@3 d@4

Example of iteration:

map = [string_map] a@1 b@2 c@3
for p: $map
{
  k = $first($p)
  v = $second($p)
}

While the subscript is mapped to key lookup only, index-based access can be
implemented (with a bit of overhead) using the $keys() function:

map = [string_map] a@1 b@2 c@3
keys = $keys($m)
for i: $integer_sequence(0, $size($keys))
{
  k = ($keys[$i])
  v = ($map[$k])
}

Also, this commit changes the naming of other template-based value types (not
exposed as buildfile value types) to use C++ template id-like names (e.g.,
map<string,optional<bool>>).

1 files changed, 21 insertions, 4 deletions

diff --git a/libbuild2/functions-string.cxx b/libbuild2/functions-string.cxx
index 8e5a315..b332efa 100644
--- a/libbuild2/functions-string.cxx
+++ b/libbuild2/functions-string.cxx
@@ -119,14 +119,16 @@ namespace build2
     };
 
     // $size(<strings>)
+    // $size(<string-map>)
     // $size(<string>)
     //
-    // First form: return the number of elements in the sequence.
+    // First two forms: return the number of elements in the sequence.
     //
-    // Second form: return the number of characters (bytes) in the string.
+    // Third form: return the number of characters (bytes) in the string.
     //
-    f["size"] += [] (strings v) {return v.size ();};
-    f["size"] += [] (string v) {return v.size ();};
+    f["size"] += [] (strings v)             {return v.size ();};
+    f["size"] += [] (map<string, string> v) {return v.size ();};
+    f["size"] += [] (string v)              {return v.size ();};
 
     // $sort(<strings> [, <flags>])
     //
@@ -204,6 +206,21 @@ namespace build2
       return find_index (vs, move (v), move (fs));
     };
 
+    // $keys(<string-map>)
+    //
+    // Return the list of keys in a string map.
+    //
+    // Note that the result is sorted in ascending order.
+    //
+    f["keys"] += [](map<string, string> v)
+    {
+      strings r;
+      r.reserve (v.size ());
+      for (pair<const string, string>& p: v)
+        r.push_back (p.first); // @@ PERF: use C++17 map::extract() to steal.
+      return r;
+    };
+
     // String-specific overloads from builtins.
     //
     function_family b (m, "builtin");