frisby | REST API framework built on Jest | Testing library

I went through this tutorial myself and I think generally functions are assumed to be curried so f being b -> a -> b is perhaps counter-intuitive but not necessarily wrong AFAIK. (Take everything I say with a pinch of salt; I'm not an expert ;)

However the parentheses around f itself in the reduce signature gives an important clue to the reader (at least to the JavaScript reader) that f is a function:

Most of encryption algorithms (I believe, HMAC included) expects data in binary. So you should .encode() key before calling hmac.new().

Wild guess – this docs on website is from py2 times and weren't updated

--watchAll option in jest Watch files for changes and rerun all tests when something changes, reference
so this command yarn test --coverage --watchAll would show coverage of all files.

I think it might depend on the language that you're using. Most languages are not equipped with a way to establish 'algebraic' equality between functions, and are likely to compare functions via their 'pointer' or 'reference', even if such is abstracted away. As a result, since your composed function would be a different function altogether (albeit with the same behaviour), it might not return as equal.

This being said, you can probably test it by having a set of input and running both functions on it, checking the equality of the output.