Why are missing free variables not detected at compile time?

[tomjaguarpaw]

I tried the following program:

partialApply :: Program
partialApply = mconcat
    [ Stg.add
    , [program|
    main = \ => let one   = \ -> Int# 1#;
                    plus1 = \x -> add x one
                in plus1 one
    |]]

It compiled and ran, ending with

7. Errorenous state: one not in scope

I should have written

plus1 = \(one) x -> add x one

Why was this not detected at compile time?

[tomjaguarpaw]

Actually, plus1 = \(one) x -> add x one doesn't seem to fix it. Can you tell me what's wrong with this program?

[tomjaguarpaw]

I had to do

partialApply :: Program
partialApply = mconcat
    [ Stg.add
    , [program|
    main = \ => let one   = \ -> Int# 1#
                    in let plus1 = \(one) x -> add x one
                       in  plus1 one
    |]]

This seems to be a consequence of "Like letrec, but the bindings cannot refer to each other (or themselves). In other words, let is non-recursive." (from the README). But shouldn't this be enforced at compile time?

[quchen]

Why was this not detected at compile time?

For two reasons,

1. STGi does not compile anything, it just takes the syntax tree as generated by the parser and puts it into the automaton
2. The only static analysis that is done (right now) is by the parser, who catches some minor mistakes such as updatable closures that don’t make any sense, as noted in your other ticket Confusing error: "Standard constructors are never updatable, expected: atom (variable or literal)" #88.

As for how to write your program, I would use a letrec.

add = ... -- Prelude
main = \ => 
    letrec 
        one   = \ -> Int# 1#;
        plus1 = \(one) x -> add x one
    in plus1 one

I’ve tried building some better static analysis a couple of times, but it’s not as trivial as I initially thought. I have a long-standing ticket open for reporting unused globals, #23, and a branch that works on that issue (static-analysis it’s called I believe).

[tomjaguarpaw]

Thanks, I like your letrec version. Static analysis to detect these errors would be good and will have a look at your branch when I get the time.

[quchen]

Is there anything else I can help you with? Otherwise I’ll close the ticket.

[tomjaguarpaw]

That's fine. You can close this one if you like.