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, 7 insertions, 3 deletions

diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index 6ccae8a..b68f009 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -6090,7 +6090,8 @@ namespace build2
           if (n[4] == 's' &&
               n[5] == '\0') return &value_traits<paths>::value_type;
         }
-        else if (n == "project_name") return &value_traits<project_name>::value_type;
+        else if (n == "project_name")
+          return &value_traits<project_name>::value_type;
         break;
       }
     case 's':
@@ -6100,12 +6101,15 @@ namespace build2
           if (n[6] == '\0') return &value_traits<string>::value_type;
           if (n[6] == 's' &&
               n[7] == '\0') return &value_traits<strings>::value_type;
+          if (n == "string_map")
+            return &value_traits<map<string,string>>::value_type;
         }
         break;
       }
     case 't':
       {
-        if (n == "target_triplet") return &value_traits<target_triplet>::value_type;
+        if (n == "target_triplet")
+          return &value_traits<target_triplet>::value_type;
         break;
       }
     case 'u':
@@ -8972,7 +8976,7 @@ namespace build2
               result = &result_data;
             }
 
-            // See if we have another subscript.
+            // See if we have chained subscript.
             //
             enable_subscript ();
             tt = peek ();