[cratermoon]

Assuming that this guide is aimed at people entirely unfamiliar with these concepts, I think it fails to explain the concept. Yes, it is plain English, but it is not simple or common English, and it's mostly code examples. I'm familiar with the concepts, but not fluent, and functional programming jargon I already knew is not clearer to me now than it was before I read it.

Good sample code, though. It only required me to look up a handful of things I wasn't already familiar with.

[b3morales]

I agree, and not just the English. There is use of particular notation that does not explain anything unless you already know what it means:

> f is a morphism from a -> b, and g is a morphism from b -> c; g(f(x)) must be equivalent to (g • f)(x)

If I don't already understand (g • f)(x) this is not helpful at all. This other one especially jumped out at me (perhaps because I've personally never seen this "bent equals sign" ≍ before):

> A functor must adhere to two rules:

> Preserves identity

> object.map(x => x) ≍ object

What does that sign ≍ mean? If I don't know, I am no closer to understanding functors. And the sign is not explained anywhere in the document.

[cratermoon]

Oh boy. I didn't put on my glasses to read it so I didn't notice that ≍ is not = and that's important.

I think in the context it just means the result is in the range of "function applied to object"

[hither_shores]

> What does that sign ≍ mean?

Normally, `f ≍ g` means `f \in \Theta(g)`. Here, the author is using it as an equals sign. I have no idea why.

[cratermoon]

Except it's not and =, it's a ≍ sign, which looks almost identical. I'm not a mathematician and I don't know the deep theory behind functional programming, but the internet tells me that ≍ in Graham, Knuth, and Patashnik's Concrete Mathematics it's defined to mean the same thing as "Big Θ"[1], as you note.

1 https://math.stackexchange.com/questions/764897/what-does-th...