How to typecheck `let`-expressions?

I’ve been thinking about the details of this. Firstly, if we don’t have let-expressions, we only have to detect all definitions at the top-level and submodules and form a dependency graph out of it. However if we have let-expressions, I think the solution is like this:

• Detect all top-level definitions and submodule definitions (but not definitions in let-expressions);
• Form a dependency graph out of them (note that if some def C in some A uses B then A depends on B too);
• When typechecking a definition and we meet a let-expression, we form a dependency graph of the module of the let-expression, typecheck them, and go on.

Why not directly form a dependency graph out of all definitions, including those in the let-expressions?

• We need to ensure that when we typecheck let-expressions, the localCtx has the variables of the outer definition.
• Hence, we should not lift the definitions of let-expressions outside like we did to modules; because this way we could not ensure the localCtx. We should typecheck them only when we meet them, at that time the localCtx will be complete.

Why won’t this cause a problem in dependency?

• If the def in let-expression depends on another definition outside, then that definition is also depended on by the outer definition of the let-expression. So when we check the let-expression, its dependencies are all checked too.
• Another definition outside cannot depend on a definition inside let-expression at all.

[ice1000]

• 👍 for having local modules
• 👎 for simple let like in Arend

@re-xyr is not allowed to cast vote

[ice1000]

Fixed!