Here are a few tips to brighten up your symbols in Emacs.

As I am using Doom Emacs, there is now a nice option to use Iosevka with full support for ligatures. Note, however, that you will need to install a patched version of Iosevka, which may not harm your system or other configuration: I use the standard Inziu Iosevka CL in a terminal, and the patched Iosevka in GUI Emacs. Once the font is installed, it is just a matter of adding (pretty + iosevka) to your init.el to get pretiffy-mode in all programming modes, with correct ligatures. The same option is available for Fira Code but I haven’t tested it, though.

Now, if you want to add some custom symbols in the prettify-symbols-alist variable like, e.g., Eric Kaschalk did (Mathematical notation in Emacs), you could simply use his approach or the one below (which is what I used to use in Spacemacs a few months ago):

(defun my/pretty-symbols ()
  (setq prettify-symbols-alist
          '(("lambda" . 955)
            ("->" . 8594)
            ("=>" . 8658)
            ("map" . 8614)
            ("<=" . 8804)
            (">=" . 8805)
            ("<-" . 8592)
            ("!=" . 8800))))
(add-hook 'clojure-mode-hook 'my/pretty-symbols)
(add-hook 'racket-mode-hook 'my/pretty-symbols)
(add-hook 'ess-mode-hook 'my/pretty-symbols)

Most of the above has become useless since modules/ui/pretty-code/+iosevka.el in your .emacs.d directory handles this for you. However, +pretty-code-iosevka-font-ligatures is adefvar, which means you cannot update its default values unless you append to it using a setq. This is what I finally did, and it went quite smoothly. I also use a nice trick found on one of Huy Tran’s websites,¹ namely replacing Org checkboxes with Unicode characters. By the way, you may also like his other posts on Emacs, for example Emacs from scratch. Other types of customization are discussed on SO.

Here is finally what I added to my config.el file:

;; Best with custom Iosevka font. See, e.g., https://is.gd/L67AoR
(setq +pretty-code-enabled-modes '(emacs-lisp-mode org-mode clojure-mode
                                   latex-mode scheme-mode racket-mode ess-r-mode))
(setq highlight-indent-guides-responsive 'top
      highlight-indent-guides-delay 0)

;; Org and R additional symbols
;; hex code ▷ (9655), ◇ (9671), ▶ (9654), ƒ (402)
(setq +pretty-code-iosevka-font-ligatures
      (append +pretty-code-iosevka-font-ligatures
              '(("[ ]" .  "☐")
                ("[X]" . "☑" )
                ("[-]" . "❍" )
                ("%>%" . ?▷)
                ("%$%" . ?◇)
                ("%T>%" . ?▶)
                ("function" . ?ƒ))))

;; https://is.gd/3VuSXj
(defface org-checkbox-done-text
  '((t (:foreground "#5a637b")))
  "Face for the text part of a checked org-mode checkbox.")

(font-lock-add-keywords 'org-mode
                        '(("^[ \t]*\\(?:[-+*]\\|[0-9]+[).]\\)[ \t]+\\(\\(?:\\[@\\(?:start:\\)?[0-9]+\\][ \t]*\\)?\\[\\(?:X\\|\\([0-9]+\\)/\\2\\)\\][^\n]*\n\\)"
                           1 'org-checkbox-done-text prepend))
                        'append)
;; (custom-set-faces '(org-checkbox ((t (:foreground nil :inherit org-todo)))))

If you want to highlight checkboxes using the same color as your TODO header, uncomment the last line.

Overall, I found that my daily log book looks much prettier now.

Likewise, here is some R code stolen from the magrittr website and the pipe chapter of R for data science – which doesn’t mean I became a fan of using pipes in R, but let’s pretend we have a decent unicode replacement for %>%, %T>%, and the like:²