The most irritating thing to me is when someone says they want to understand something, but then they are unwilling to put in any real effort. It is a misapprehension, misunderstanding and misrepresentation to say:
... reinventing the wheel runs counter
to everything we do in CS.
You can read all you like about how to juggle - if you don't put in the hours, you won't be able to do it.
You can read all you like about how to unicycle - if you don't put in the hours, you won't be able to do it.
You can read all you like about how to program - if you don't put in the hours, you won't be able to do it.
Skills require practice. These instances of "left as an exercise for the interested reader" are to help you really learn and properly understand the material. And if you don't care, don't bother. If you are unwilling to put in the time then the chances are that you would end up thinking you understand an explanation, whereas in fact you don't.
The above is only for good writing, of course. There are plenty of instances of bad writing, but that's not then a complaint about the subject, it's a complaint about the writer.
Someone once said of Feynman that his lectures were beautifully clear, and that those who listened gained real insight and understanding. Until they had to use it. Then they realised that they didn't really understand it at all.
If you want understanding, do the exercises.
So there you are, I've done as you predicted and pointed out that you're just being lazy, and that if you really want to learn then you have to put in the work. Just because you preempted it, doesn't mean it's wrong. It's right, you already know it. Complaining won't help you.
You must have only seen this phrase in places where it was used in the bright, sparkling, professorial manner assumed by your comment. I take no offense when it appears in a textbook, where the intent is to have me practice a skill. I do object to its appearance in a journal article, however; that is a context where you are supposed to communicate findings as clearly and concisely as possible. In particular, I've come across its use more and more often where one of the following likely applies:
1. it is doubtful that the author has actually done the work to prove his own assertion, and uses the phrase to cast his own burden on the reader,
2. the author has possibly done the work, but can't be bothered to condense it to the quality required for publication,
3. showing the work would clearly be useful for the target audience, but the author is more concerned with making the material appear difficult, or
4. the author is being ironic, because the assertion is either superfluous, outright incorrect, or known to be unprovable.
tldr: I am indeed complaining about bad writing, not the subject--specifically that "left as an exercise" has become a common idiom behind which bad writers in math and CS hide, and its inappropriate use is now all too common.
Yes, it's used too often. Yes, it's used sometimes when it's inappropriate. Yes, sometimes I suspect that the author doesn't really know the answer. I'm less sure that the author uses it it all seriousness when the know the result is unproven or unprovable.
But it is a phrase that has its place, there are times when it's exactly right to use it. Don't reject it outright. I've used it in publication where the editor actually asked me to remove details and put the phrase in its place.
You can read all you like about how to unicycle - if you don't put in the hours, you won't be able to do it.
You can read all you like about how to program - if you don't put in the hours, you won't be able to do it.
Skills require practice. These instances of "left as an exercise for the interested reader" are to help you really learn and properly understand the material. And if you don't care, don't bother. If you are unwilling to put in the time then the chances are that you would end up thinking you understand an explanation, whereas in fact you don't.
The above is only for good writing, of course. There are plenty of instances of bad writing, but that's not then a complaint about the subject, it's a complaint about the writer.
Someone once said of Feynman that his lectures were beautifully clear, and that those who listened gained real insight and understanding. Until they had to use it. Then they realised that they didn't really understand it at all.
If you want understanding, do the exercises.
So there you are, I've done as you predicted and pointed out that you're just being lazy, and that if you really want to learn then you have to put in the work. Just because you preempted it, doesn't mean it's wrong. It's right, you already know it. Complaining won't help you.
Put in the time.