fatcerberus

Monads for the Rest of Us

by Bruce Pascoe - 1 May, 2019

"A monad is just a monoid in the category of endofunctors. What's the problem?" ~James Iry[^1]

The problem... is that there are several problems.

It's been said that monads bear a dreadful curse. Once you finally understand what they are, you begin to see them everywhere--but somehow become completely incapable of explaining them to anyone else. Many tutorial writers have tried to break the Great Curse--the Web is lousy with bold attempts and half successes that attest to this--and just as many have failed. Well, I'm here to address the elephant in the room[^2] and tell you that I intend to break the Great Curse once and for all.

There are basically two ways a monad tutorial tends to go. One is a paragraph or two of minimal descriptions of one or two common monads (Haskell's Maybe in particular is very popular), followed by a lot of intimidating Haskell syntax trying to explain--precisely--how it all fits together. This is well

Avaq

const I  = x => x
const K  = x => y => x
const A  = f => x => f (x)
const T  = x => f => f (x)
const W  = f => x => f (x) (x)
const C  = f => y => x => f (x) (y)
const B  = f => g => x => f (g (x))
const S  = f => g => x => f (x) (g (x))
const S_ = f => g => x => f (g (x)) (x)
const S2 = f => g => h => x => f (g (x)) (h (x))

vitorgalvao

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