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No golfing language has shaped Code Golf Stack Exchange leaderboards more than Jelly, created in 2015 by Dennis Mitchell, the site's most decorated golfer. Jelly is tacit: programs are chains of links, function atoms composed without ever naming arguments, an idea inherited from the J array language. It uses a custom 256-character codepage so every atom costs one byte, and its dictionary-backed string compression turns whole phrases into a few characters. For years the phrase written in Jelly at the top of a leaderboard was simply expected. The interpreter is written in Python, and XCODX runs it behind a real terminal, so you can paste codepage programs, supply stdin interactively, and watch results without any local Unicode wrangling.
“Hello, World!”
How it works: the quoting characters delimit a Jelly string literal, and a full program's result is printed implicitly, so this one-link chain outputs the greeting. Golfed versions go further, using Jelly's dictionary compression to encode both words in fewer bytes than the raw text.Jelly is the reference tool for serious code golf: arithmetic, combinatorics, base conversion, and matrix challenges routinely collapse into chains of a few atoms, and studying top-voted Jelly answers is how many golfers learn advanced techniques. It also rewards the intellectually curious beyond competition, because writing tacit chains forces a different mental model, composing functions rather than shuffling variables, which makes it a compact playground for point-free programming ideas.
Programs never name their arguments. A Jelly line is a chain of atoms, and fixed rules decide how each atom receives the chain's left and right arguments based on its arity and position. This mirrors trains in the J language. Once internalized, the rules let two or three atoms express what an explicit lambda would spend many characters declaring.
Jelly defines a 256-character set where every symbol maps to exactly one byte, so a program using characters like the quoting marks or dotted letters is scored at one byte each rather than their multi-byte UTF-8 size. Code Golf Stack Exchange accepts scoring in a language's documented codepage, and this convention is standard across Jelly, 05AB1E, and Vyxal.
They are perennial rivals with different flavors. Jelly leans mathematical and tacit, strong on number theory, arrays, and link manipulation; 05AB1E is stack-based with an enormous library of pragmatic built-ins and excellent string compression. Leaderboard results split by problem domain, and many top golfers keep both languages in rotation, choosing per challenge.
main.jellyplain text — no highlighter ships for this languageno standard input construct“Hello, World!”