HAhardwaresofton·Jul 5, 2019
I really support this kind of writing, I don't think this kind of stuff is written about enough, and it's exactly what everyone learns the hard way working on software projects. Skill wise, knowing the kinds of reasoning/techniques that articles like these discuss is the difference between "junior" and "senior" developers (though I very much dislike those terms).
One thing I want to point out -- if at all possible do not use decimals for money:
> We could use decimal (and we will, but no directly), but decimal is less descriptive. Besides, it can be used for representation of other things than money, and we don't want it to be mixed up. So we use custom type type [<Struct>] Money = Money of decimal .
The custom type is a great idea (try to write code in languages that make this concept easy, I suggest Haskell & type/newtype). The problem
Read the rest
here is that decimal is the wrong type for storing money[0]. Your first IEEE754 floating point bug teaches you this, but in general trying to write code around manipulating decimals can get very messy really quickly when precision is involved in any case. Another example is JSON, JSON numerics are actually all floats under the covers, so this means if you store more precision or a bigger number than it can handle, things can get wacky if you're not careful -- this is one of the places where being "stringly typed" (and defining your own unpacking to go with your domain types) can be very helpful.
Libraries like dinero.js[1] exist because of how surprisingly hard this problem is, kind of like how moment[2] exists due to how hard dealing with time can be.
[0]: https://stackoverflow.com/questions/3730019/why-not-use-doub...
[1]: https://github.com/sarahdayan/dinero.js
[2]: https://momentjs.com