Plain English description of monads without Haskell code

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Monads are notorious in the programming world for their use in the Haskell programming language and for being hard to grasp. There’s even a joke that writing a “monad tutorial” is a rite of passage for new Haskellers, and been described as pointless. By Chris Done.

One of the Haskell designers3 in the 90s just came up with a class/interface that worked for all of these. As he was into category theory, he “monad”. The types also sort of match the theory if you squint hard enough.

You can re-use your intuition from existing common place chaining of things in other popular languages:

  • Async chains (JS)
  • Parser combinator chains (Rust, JS)
  • Optional or erroneous value chains (TypeScript, Rust)
  • Continuation passing style (you can do this in Lisp and JS)
  • Cartesian products/SQL (C#’s LINQ)

Monad is the name of the class for “and_then”,5 defined in a sensible way, with some laws for how it should behave predictably, and then a bunch of library code works on anything that implements “and_then”. Apart from F# or Haskell (or descendants), no other language embraces the abstraction with syntax so it’s hard to find a good explanation without them. It’s like explaining Lisp macros without using a Lisp, the explanation tends to be awkward and unconvincing.

Haskellers don’t like to throw exceptions, or use mutation, and functions can’t return early, etc. Suddenly Monad and syntactic sugar for it looks pretty attractive to them. Nice one!

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