My intention is not to write code that will go wrong at some point. My intention is to understand the inner workings of the compiler, apparently did not explain me well.
Let me explain:
If I use a function that should always return 5, and for unknown reasons returns 8. I wonder because it failed and so help with a solution.

Anyway, if you do not understand me, I will need repeat my studies of English, or no longer use google translator. :)

Do not worry I understand, what I say.

Thanks.

My intention is to understand the inner workings of the compiler

I agree with you that such knowledge is imponderably useful. Any help information should explain this in detail. Then you can predict much easier what happens in the background and decide what kind of data types have what advantages/drawbacks.

But if higher level functions (like Length) exist that can be used without any performance penalty (and even if it makes the program a *little* bit slower) it is wise to use them. Any changes that are done "under the hood" later will not harm your program.