February 6, 2026
Monads, memes, mild mayhem
The Monad Called Free
Haskell cult classic sparks nostalgia—and cries for simple examples
TLDR: A classic Haskell blog shows the Free monad acting at a higher level, sparking both cheers and confusion. Fans gush about stability and theory while newcomers demand practical examples and point to a “freer” variant—proof that elegant ideas still need real-world demos to win hearts.
A deep-cut Haskell post just resurrected the “Free monad” and showed it’s not just a programming tool, it can itself act like a higher-level monad—think lego bricks that build other sets of lego bricks. The code dives into functors (containers) and natural transformations (shape-shifting between containers) to show how Free fits into a meta-world of “monads of monads.” Nerdy? Extremely. But the crowd? Loud.
Long-time fans turned the thread into a reunion tour. One commenter practically threw confetti: “I owe so much to this blog!” and linked the legendary Loeb function, a brain-bender that Haskell veterans love to flex. Meanwhile, pragmatists pushed back hard: “I keep bouncing off this stuff… where are the concrete examples?” Cue the classic Haskell split—devotees celebrate purity and theory while newcomers want a cookbook, not a thesis. Hot takes rolled in about the “kmettoverse” staying “stable” (aka unchanged for years), sparking a side debate: stability vs. stagnation. Another thread went full spinoff, hyping the “freer” monad as the edgy reboot based on “bind” instead of “join.” Jokes flew: “Design Patterns for Monads when?” and “Can a monad do my taxes?” Even LLMs got dragged in—apparently decent at debugging Haskell, which somehow made everyone both amused and a tiny bit terrified. High theory, low patience, maximum drama.
Key Points
- •The article frames the Free construction as a higher-order monad in the category of endofunctors (Endo), not just a standard Haskell monad.
- •It presents the standard Haskell definition of Free with corresponding Functor and Monad instances.
- •Endo is defined with functors as objects and natural transformations as arrows, represented in Haskell via a RankNTypes-based Natural type.
- •An HFunctor type class is introduced to formalize functors in Endo, with ffmap for value mapping and hfmap for natural transformations.
- •Endo analogs of products (via functor composition), sums, and list/Monoid structures are discussed, noting non-commutativity and transfer caveats from Hask.