MyParser isn’t a variable. It’s a module name (module names are also atoms).
The docs for nimble_csv show that NimbleCsv.define/2 actually defines a new module, using the name you provide (in the example, its MyParser) that implements a parser using the options you pass.
Thats why you call MyParser.parse_string/1 to do the actual parsing of some CSV data. MyParser is just a regular Elixir module that just happened to be generated for you.
