Reporting errors from the compilation phase #

Latest changes to Valeri have introduced significant improvements to error reporting, not only within the parser, but also within the compiler. Where previously the compiler will error out with a cryptic "compilation error", it now explains what went wrong and where.

As an example, let's input an incorrect expression in the REPL:

valeri> (let (("x" 1)) x)
<stdin>:1:8 Syntax error: Binding name must be a symbol

As you can see, it shows you that the error happened in <stdin> at a specific location, and that it is because you specified a variable binding that is not a symbol.

The same happens when you write the expression to the file, and call the file. But additionally it will show you the filename:

./valeri error_report.vli
error_report.vli:1:8 Syntax error: Binding name must be a symbol

There aren't that many syntax errors that you can make in a real program, because only special forms get this type of treatment (such as function definitions, conditions, comparison, etc). But it's already quite a bit better than what you may find in other Lisp flavors. For example, if you do this in Guile:

scheme@(guile-user)> (let (("x" 1)) x)
While compiling expression:
Syntax error:
unknown file:1:0: let: bad let in form (let (("x" 1)) x)

Alright, it is a "bad let", but why? I guess the poor error reporting in this case is due to the fact that in Scheme a lot of basic operations like "let" are actually "lowered" to more primitive things such as lambdas. And this lowering is performed by macros, and not by special-purpose handlers in the compiler.

I'm sure the error reporing will become 10x harder when I make it to implementing macros. But as it stands right now, I'm happy with the current implementation for the purpose of writing simple programs.