Vim autocomplete to store words between documents

Question

User statement:

I use Vim to write many technical text documents, including emails.

Question:

Is it possible to "store unique words" from past documents in some .vim/file that would be automatically loaded when any future file is opened and the Ctrl+N command is issued? I know others have asked about pointing to a dictionary file in addition to the current file (e.g., https://stackoverflow.com/questions/20887628/vim-auto-complete-both-with-current-file-and-dictionary) but I'd rather the process was dynamic.

Ideal behavior:

I'd like to write one email containing Supercalifragilisticexpialidocious. Then, in some future email, I'd like to start writing Superc and then Ctrl+N would complete the word.

Further thoughts/requests:

1. I assume there is concern about such a "carryover file" growing excessively large over time. It would be great if it is possible to prevent "words shorter than K characters" from being stored.
2. I'm also willing/interested in manually editing such a file.

Answer 1

As far as I know, the best you can do in this regard is to build a custom dictionary manually and include those new words as part of the matches for Insert mode completion.

In your runtime user directory, create a new directory called spell and in your .vimrc, add the following lines:

set spell
set spellfile=~/.vim/spell/vim-spell-en.utf-8.add
set complete+=kspell

You may want to read :h 'spellfile' and :h spell-load, but basically, vim will store your "custom" words in the path set by spellfile (this file must end in .{encoding}.add). If you only want to use this custom dictionary for a single language, add a prefix for the language as shown above ({language}.{encoding}.add).

After adding your words (with zg), run set spell, type the first letters of your word and press Ctrl+N and you word will be among the suggestions in the pop-up menu.

On your additional requests:

1 - That should be possible by calling a function that

• List all the words in a buffer (see https://stackoverflow.com/a/48024588/460147)
• Filter such words based on your condition (:h filter())
• Loop over those words with execute ':spellgood! ' . word

2 - Unfortunately, you cannot manually change this dictionary since this procedure results in a binary file.