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, 10 insertions, 7 deletions

diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index 6b0b30e..d67098e 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -111,11 +111,11 @@ namespace build2
     //
     // Note: should normally be consistent with iterate.
     //
-    value (*const subscript) (const value& val,
-                              value* val_data,
-                              value&& subscript,
-                              const location& sloc,
-                              const location& bloc);
+    value (*/*const*/ subscript) (const value& val,
+                                  value* val_data,
+                                  value&& subscript,
+                                  const location& sloc,
+                                  const location& bloc);
 
     // Custom iteration function. It should invoked the specified function for
     // each element in order. If NULL, then the generic implementation is
@@ -1181,8 +1181,11 @@ namespace build2
   //
   // Either K or V can be optional<T> making the key or value optional.
   //
-  // Note that append/+= is non-overriding (like insert()) while prepend/=+
-  // is (like insert_or_assign()).
+  // Note that append/+= is overriding (like insert_or_assign()) while
+  // prepend/=+ is not (like 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 representation (e.g.,
+  // a@0 a@1).
   //
   template <typename K, typename V>
   struct map_value_type;