[treyfitty]

> I don’t complaints about syntax error for lines I haven’t finished writing

How about “complaints about” syntax errors after you’ve finished writing?

Jokes aside, to answer why knowing the value of ‘a’ might be useful before writing the full equation: ever work in excel and you’re building up a calculation, and when finished, your answer isn’t what you expected? Tracing that missing parentheses or brain-fart typo can be frustrating. I imagine that’s the same reason why some people would care about the value of ‘a’ before they’ve finished.