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, 2 insertions, 2 deletions

diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index 6cdf3ee..40eeccd 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -1994,7 +1994,7 @@ namespace build2
     {
       // Seeing that we are reversing for consumption, it feels natural to
       // reverse JSON null to our [null] rather than empty. This, in
-      // particular, helps nested subscript.
+      // particular, helps chained subscript.
       //
 #if 0
     case json_type::null:               r = value (names {});          break;
@@ -2085,7 +2085,7 @@ namespace build2
              ? json_subscript_impl (val, val_data, i, n, index).first
              : value ());
 
-    // Typify null values so that we get called for nested subscripts.
+    // Typify null values so that we get called for chained subscripts.
     //
     if (r.null)
       r.type = &value_traits<json_value>::value_type;