blk

blk tries to generalize the idea of creating and navigating titles of text files, as well as making links from one file to another, or one block to another (possibly in different files). if you have used org-roam, denote, or other similar tools like obsidian and logseq, you would know that inserting links between two files (or nodes, as they're usually called) and navigating them is a must-have feature for note-taking, with blk, instead of restricting links to specific elements of text, such as a file or a heading, we can insert links to arbitrary forms of text, be it links to an org heading (or markdown heading), or a code block in an org file, or even links to python functions, elisp functions, or an html element (by its id).

most note-taking packages introduce rules or syntaxes of their own, for example, org-roam inserts something akin to the following text to identify headers and files (which, by the way, blk knows how to handle):

:PROPERTIES:
:ID: <some-random-id-here>
:END:

while this may be sufficient for most, it wasnt flexible enough for my use case, what if i wanted to identify a specific code block by a unique string like org-roam does for headers and files? what if i wanted to identify a list item? say i wanted to write down the axioms of a vector space and i do it in the following manner in my org-mode file:

#+begin_definition
a /vector space/ is a set that satisfies the following axioms:
- closure of addition,
- closure of multiplication
- closure of..
- etc..
#+end_definition

so far, so good. but what if i wanted to write the following text in another org file:

consider the second axiom of vector spaces, we..

shouldnt the text second axiom be clickable, and shouldnt it redirect the user to the third axiom of the definition we have above when clicked? an org-roam user would most likely insert a link to the header or the file containing the definition, but this isnt precise enough. first, it doesnt send the user to the axiom itself. second, the file may contain other text which would make finding the axiom the text refers to even less trivial.
to accommodate this, blk suggests the following syntax:

#+begin_definition
a vector space is a set that satisfies the following axioms:
- closure of addition,
- <<<vecspace-axiom-2>>>closure of multiplication
- closure of..
- etc..
#+end_definition

this is what the org mode documentation refers to as a named target. org mode by default allows linking to a named target using the text [[named-target-here]], so for the target above, we would write:

consider the [[vecspace-axiom-2][second axiom]] of vector spaces, we..

but as one might guess, this only works when the both the definition and the link exist in the same file, what blk suggests is simply the following:

consider the [[blk:vecspace-axiom-2][second axiom]] of vector spaces, we..

this way, no matter where you place the link, it would work, in another org file, in another markdown file, or in any other type of text file you would want (as long as a rule is defined for it, the predefined rules should cover some common use-cases such as this).
again, so far so good, but the text vecspace-axiom-2 acts as an "id" and may be hard to remember each time you want to pull up the definition of a vector space. so again, to accommodate this, blk suggest to rewrite the definition block as the following:

#+name: vecspace
#+begin_definition :title vector space
a vector space is a set that satisfies the following axioms:
- closure of addition,
- <<<vecspace-axiom-2>>>closure of multiplication
- closure of..
- etc..
#+end_definition

better yet, the #+name property can be written on the same line as the title property:

#+begin_definition :title vector space :name vecspace
a vector space is a set that satisfies the following axioms:
- closure of addition,
- <<<vecspace-axiom-2>>>closure of multiplication
- closure of..
- etc..
#+end_definition

in both cases, blk-find can be invoked, and the user can navigate to the definition by its title "vector space", and if the user wants, they can even rewrite the cross-reference of the text we considered before to link to the definition too, in the following manner:

consider the [[blk:vecspace-axiom-2][second axiom]] of [[blk:vecspace][vector space]]s, we..

or, if the user enables blk-treat-titles-as-ids (set to t), they can write the following:

consider the [[blk:vecspace-axiom-2][second axiom]] of [[blk:vector space]]s, we..

when opened, both links would redirect to the second axiom, and the definition, respectively. (notice that the keywords :title and :name are supported for any type of block, not only #+begin_definition blocks, so we can use them for a #+begin_theorem, for example.)
again, so far so good. but what if we want to write some code that makes heavy use of mathematics, and for some reason we wanted to refer to the definition of a vector space in our code, how would we do this? simple enough:

(defun my-function ()
  "this function does the following etc... refer to the definition of a [[blk:vector space]]"
  (message "magic!"))

as expected, when the function blk-open-at-point is invoked at the link, emacs takes us to the definition of a vector space.
notice that for this to work, blk has to find the mentioned files in the directories listed in the variable blk-directories (it would search recursively if blk-search-recursively is set to t).
we can take things even further and use transclusions. say we want to not just link to the definition, but to insert it into the current document, without having to copy it from the other file and paste it into the current document, which would mean that modifiying one of them would cause the other to be "outdated", this is done using blk's integration with org-transclusion:

consider the following definition of a vector space:
#+transclude: [[blk:vecspace]]

to expand the transclusion we can use M-x org-transclusion-mode.
this workflow is demonstrated in the following gif:

• linking to any type of text, without having to worry about filenames or the location of the text itself.
  
• navigating text elements by their titles, including support for outline navigation, e.g. the outline for an org src block: file/heading/block.
  
• support for alises to any title, so that any text element can be navigated to by its "main" title or one of its aliases (this includes org files, for an example).
  

(use-package blk
  :straight (blk :host github :repo "mahmoodsh36/blk") ;; replace with :quelpa if needed
  :after (org)
  :config
  (setq blk-directories
        (list (expand-file-name "~/notes")
              user-emacs-directory))
  (add-hook 'org-mode-hook #'blk-enable-completion)
  (setq blk-use-cache t) ;; makes completion faster
  (global-set-key (kbd "C-c o") #'blk-open-at-point)
  (global-set-key (kbd "C-c f") #'blk-find)
  (global-set-key (kbd "C-c i") #'blk-insert))

if you want the transclusion functionality, you have to enable it after installing org-transclusion:

(use-package org-transclusion
  :config
  (add-hook 'org-mode-hook #'org-transclusion-mode))

(use-package blk
  :straight (blk :host github :repo "mahmoodsh36/blk") ;; replace with :quelpa if needed
  :after (org org-transclusion)
  :config
  (setq blk-directories
        (list (expand-file-name "~/notes")
              user-emacs-directory))
  (add-hook 'org-mode-hook #'blk-enable-completion)
  (blk-configure-org-transclusion)
  (setq blk-use-cache t) ;; makes completion faster
  (global-set-key (kbd "C-c o") #'blk-open-at-point)
  (global-set-key (kbd "C-c f") #'blk-find)
  (global-set-key (kbd "C-c i") #'blk-insert))

when im taking notes in math, what i like to do is write the following:

#+begin_definition :name def-comp-func :defines computable function :alias partially computable function
a /computable function/ is any function that is a member of the class of mu-recursive functions.
#+end_definition

in my mind, :defines tells me what this block defines, :alias tells me that the following value is an alternative name to what :defines has, and :name is supposed to give the block a unique string it is identified by.
now, with the rules pre-defined in blk.el, it would be possible to use the keyword :title and have blk recognize it, but not :defines. to have blk recognize this new keyword, we add the following rule, which is a modified version of the rule blk-rg-org-file-rule from blk.el:

(add-to-list
 'blk-patterns
 (list :title "definition"
       :glob "*.org"
       :anchor-regex "(:defines)\\s+[^:]+"
       :title-function 'blk-value-after-space-upto-colon
       :extract-id-function 'blk-org-id-at-point))

notice that this assumes that blk-grepper is set to blk-grepper-rg or blk-grepper-grep (this is true by default, depending on whether you have rg installed, see the different greppers section).
the value provided to :anchor-regex may look scary to someone who isnt very familiar with unix regex, but its really just a regex passed to the grepper to identify the keywords we're aiming for.
the roles of the keywords in the list above are described in the docstring of blk-patterns:

The list of patterns to use with the blk grepper.
Each entry should be a plist representing the data of a pattern:
  :title is the title/type of the pattern (used for completing-read).
  :glob is the glob pattern to matches files to be grepped.
  :anchor-regex is the regex for matching blocks of text that contain
    the target value which is then passed to :title-function to be
    turned into the final desired value to be passed to completing-read
    to serve as the entry in the completing-read menu for the target.
  :src-id-function is the function that gets the id to be used when
    creating links to the target; the need for :src-id-function over
    :title-function is that an id and a name/title for a target can be
    different, as an id can be a random sequence but a name could be
    a more memorable sequence of characters.  the function takes the matched
    value and strips unnecessary turning it into just the id,
    think \label{my-id} -> my-id
  :title-function is a function that takes as an argument the matched
    text and extracts the title from it.
    think "#+title: my-title" -> "my-title".
  :transclusion-function is a function that should take
    the match and return an object or plist that can be handled by
    org-transclusion, this allows for easily defining custom transclusion
    functions for different patterns of text.  see the function
    `blk-org-trancslusion-at-point' for an example.
  :extract-id-function is a function that, given a title, opens the
    destination entry using the gathered metadata and grabs the id
    that corresponds to a particular entry.  for example, given a
    result that matched the title of an org file, this function
    is called after opening the org file, the grep-result is passed
    to it, it should return the id of the org file that was opened.
    this is needed because when grepping we cant tell which id is
    is associated with which title (even if they're in the same file
    or belong to the same portion of text).

the greppers available for use are currently the standard grep or ripgrep (rg), or emacs itself. only use emacs as the grepper if you really are trying to avoid the dependency of an external grepper as it is an order of magnitude slower than the other options, though it is good to note that the plus side of using emacs as the grepper is that it is aware of unsaved changes to buffers since it greps those instead of the files themselves when they're already opened in buffers.
for each grepper a different table of patterns is defined, the grepper is chosen by setting the variable blk-grepper and defaults to rg and falls back to grep if rg isnt installed, and falls back to using emacs if grep isnt found aswell.

• blk-grepper-rg blk-rg-patterns
  
• blk-grepper-grep blk-grep-patterns
  
• blk-grepper-emacs blk-emacs-patterns
  

the rules that are passed to the grepper are stored in blk-patterns, which initially equals one of the variables listed above, but may differ if the user modifies it directly.

blk provides the function blk-all-to-json to dump the data that it gathers from files into a json file, for external use.

the following variables can be used to change some behaviors in the package.