Checks to see if the input token list is a member of the language.

Calculate the Brzozowski derivative of this language with respect to the given string. This is an effectful function because it can call PrimType.validate (e.g., when validating file paths, etc.).

Returns a set consisting of the first token of all strings in this language that are useful for CLI tab completion. For infinite or unwieldly languages, it is perfectly fine to return the empty set: This will simply not display any completions to the user.

If you'd like the cursor to advance to the next word when tab completion unambiguously matches the prefix to a token, append a space (" ") character to the end of the returned token. Otherwise, the cursor will skip to the end of the completed token in the terminal.

Some examples of different use cases:

1. Completing file/directory names:
   • Append a space to the ends of file names (e.g., "bippy.pdf"). This is because we want the cursor to jump to the next argument position if tab completion unambiguously succeeds.
   • Do not append a space to the end of a directory name (e.g., "foo/"). This is because we want the user to be able to press tab again to gradually complete a lengthy file path.
   • Append a space to the ends of string tokens.

You may be asking why we don't try to use the -o nospace setting of compgen and complete. The answer is they appear to be all or nothing: For a given tab completion execution, you have to choose one behavior or the other. This does not work well when completing both file names and directory names at the same time.

This is the delta (δ) predicate from "Parsing With Derivatives", indicating whether this language contains the empty string.