Rule idea: check map field updates

[erszcz]

In order to avoid a schema-less "data bag" situation when using maps, it seems to be a good practice to use #{a => A} only in one place when initially creating a map. Fields expected to be used in the future should be explicitly assigned undefined (or some other sentinel value) to mark that their presence is expected. After that only #{a := A'} should be used for updating values of existing fields and => should not be used again. That way for each "kind" of map there's one place in code defining what fields should be present in the map.

The Elvis rule would find all map update operations (i.e. M#{a := A'}) and ensure none of them use => for updating the map.

[paulo-ferraz-oliveira]

It seems some of the things you refer can be dealt with by dialyzer already, namely the expected content/format of a map. If you want a compulsory field set as undefined, you can use notation #{ FieldName :: binary() := undefined | pos_integer()}, for example.

What kind of problem are we trying to solve here?

[elbrujohalcon]

This was written before dialyzer had support for defining strict map types like…

-type my_particular_map() :: #{a := pos_integer(), b := string()}.

I'll close this one. @erszcz: feel free to reopen it if you still want an elvis rule for this.