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lexical-binding: t in all filescl-lib functions (cl-loop, cl-defun, cl-destructuring-bind) over legacy cl.elpcase for pattern matching over nested condwhen/unless over single-branch ifseq.el for sequence operations when appropriatepackage-name-function-namepackage-name--internal-functiondefcustom inside a defgroup(make-hash-table :test 'equal), puthash, gethash, maphash(plist-get plist :key), (plist-put plist :key val)(alist-get key alist), (assoc key alist)(with-temp-buffer ...) for string processing(with-current-buffer (process-buffer proc) ...)declare (indent N) for proper indentation(doc-string N) when the Nth arg is a docstring`(progn ,@body)eval — prefer macros for code generationcondition-case for catching errorsuser-error for user-facing errors (doesn't trigger debugger)error for programming errorswith-demoted-errors in hooks/advicesave-excursion when moving point temporarilyinhibit-read-only via let for modifying read-only buffers(point-marker), (set-marker-insertion-type m t)(propertize text 'face 'font-lock-keyword-face)(ert-deftest test-name () ...)(should ...), (should-not ...), (should-error ...)(let ...) for test isolation — rebind globals(make-temp-file "prefix") + (delete-file f) in cleanuplist* is NOT available — use (append (list a b) rest) insteadthread-first / thread-last available but less idiomaticfboundp before calling optional features