Support "if - not case" statements

[hydro63]

Problem

I have been recently doing some web scraping and validating configuration json, and i wanted to use pattern matching to make it easier. The problem arised when i needed to do multilevel configuration validation, and i found out that there is no real way return when configuration is wrong, meaning i have to nest multiple nseted if-s for easy validation

Map<String, dynamic> configuration;
...
List<NovelData> search(...){
  if(configuration case {
    "search": {
      "site": String url,
      "name_key": String name_key,
      "pagination_key": String page_key,
      "items_container": var item_configuration
    }
  }){
    if(item_configuration case {
      "novel_name": String name_selector,
      "novel_link": String link_selector,
      "image_link": String image_selector,
      "author": String author
    }){
      // do something with author
    } else if (item_configuration case {
      "novel_name" : String name_selector,
      "novel_link": String link_selector,
      "image_link": String image_selector,
      // field "author" is optional;
    }) {
    } else {
        return [];
    }
  }
  return [];
}

It's very longwinded, and the more layers there is to validate, the more ifs are nested inside, because there is no real way to quickly return if the pattern doesnt match.

Proposal

I propose adding if - not case statement, which will run if the pattern doesn't match or when the when doesn't pass. If the pattern matches, all the variables will be declared and ready to use on the outside. Here is my proposed syntax:

if(variable !case pattern when ...) { return ... } // based on != operator
if(variable case! pattern when ...) { return ... } // based on is! operator, not a fan of this
if(variable not case pattern when ...) { return ... } // the most readable option, but additional keyword

My code using the proposed syntax:

List<NovelData> search(...){
  // dont have basic config
  if(configuration !case {
    "search": {
      "site": String url,
      "name_key": String name_key,
      "pagination_key": String page_key,
      "items_container": var item_configuration
    }
  }) return [];
  
  // have basic config, dont have complex config
  if(item_configuration !case {
    "novel_name": String name_selector,
    "novel_link": String link_selector,
    "image_link": String image_selector,
  }) return [];
  
  // have complex config, dont have author
  if (item_configuration !case {"author": String author}){
    ...
  }
  
  // have everything
  ...
  return [];
}

This way, my code is a lot easier to navigate and understand. While this example may not seem big, often times we have to validate 3 or more layers with many optional fields. In such cases, having a syntax for quick return is a big improvement.

[mateusfccp]

Related: #2537.

[hydro63]

Issue closed due to duplicity - see #2537