Appeal to Monads

Published on 08-03-2016

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Fallacy: An Appeal to Monads

…In terms of the thought process behind how these structures are implemented I believe some similarities can be drawn, which does give me the impression that idea is not necessarily a bad one. In that same way that moving closer to the equator typically implies warmer weather.“Monads” and the continuous spectrum of goodness proximity.

The Appeal to Monads is a form of logical fallacy which creeps up in programmer discussions. Typically, a programmer presents some data structure and claims it is like a Monad, and is therefore good. Here, I demonstrate the argument is rubbish.

Form of fallacy

For some program, X, the following argument is made:

Premise 1. X is Y

Premise 2. Monads are Y

Premise 3. Monads are Good.

Conclusion. Therefore, X is Good.

Premises 1 and 2: X is Y and Monads are Y

This is a formal way of saying “X is like a monad”. Y is (presumed to be) the set to which some or all monads belong. For X also to be a member of Y, it must share some property with the Monad.

Typically this property is a non-idiomatic form of error handling, or other control flow, that is said to somehow be like Haskell monad code.

Premise 3. Monads are good.

Monads are a form of abstraction that requires parametric polymorphism. If your language does not support parametric polymorphism, you won’t get a monad.

Imagine using a monad in a language that allows arbitrary mutation of state. Your monad tries to make some guarantee over its computation. At some point, a call is made to external code. Something happens which you were trying to prohibit. What is the point?

Unless you write in a functional language that has:

There is little utility to be found using monads: in your case monads are bad.

Conclusion. Therefore, X is good.

Even if monads are good, your thing is not a monad. Perhaps it is superficially like some type of monad. But it does not follow that it will be good; it does not follow that it would be good for the same reason that the monad was good.

A counter example.

Handling uranium with your ass is kinda like what happens at a nuclear power station.

I mean, nuclear engineers use gloves and have a point to what they are doing, but never mind that.

Nuclear power stations are legit.

Therefore, sticking uranium up your ass is legit.

Everyone agrees that nuclear power is legit, right?

How to avoid making this mistake

Examples in the wild


Because monadic error handling will certainly prevent your PHP code from breaking.

Taking Monads to OOP PHP

Bonus: begins with For now, let’s not worry too much about what a Monad is.


If it looks like a duck, and walks like a duck, it’s probably an ephemeral construct of category theory.

Refactoring Ruby with Monads

Because Pipelining makes Promises easy. My pipelining code is like a Monad. Monads are good…

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