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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>>).
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