Support for macros to reduce code duplication

[segeljakt]

Hi, currently when using egglog there is some code duplication in my programs. For example:

(push)
;; 2 * (x + 3) = 6 + 2 * x
(let a0 (Mul (I64 2) (Add (Var "x") (I64 3))))
(let b0 (Add (I64 6) (Mul (I64 2) (Var "x"))))

(run-schedule
  (seq
    (run desugar)
    (saturate (run arith))))
(check (= a1 b1))
(pop)

(push)
;; 6 + 2 * 3 = 12
(let a1 (Add (I64 6) (Mul (I64 2) (I64 3))))
(let b1 (I64 12))

(run-schedule
  (seq
    (run desugar)
    (saturate (run arith))))
(check (= a0 b0))
(pop)

If I am not missing something, it is currently not possible to parameterise commands/expressions. The only workaround I see is to write a program in Rust/Python which calls into Egglog.

Would it be possible to add basic support for macros that rely on textual substitution in egglog? This would allow reusing commands such as:

; Define a macro which takes 2 parameters e0 and e1
(macro test (e0 e1)
  ((push)
   (let a e0)
   (let b e1)

   (run-schedule
     (seq
       (run desugar)
       (saturate
         (run-schedule
           (seq
             (run desugar)
             (saturate (run arith))))
   (check (= a b))
   (pop)))
   
; Use the macros
(call-macro test
  ((Mul (I64 2) (Add (Var "x") (I64 3)))
   (Add (I64 6) (Mul (I64 2) (Var "x")))))
  
(call-macro test
  ((Add (I64 6) (Mul (I64 2) (I64 3)))
   (I64 12)))

[oflatt]

I agree that macros would be quite useful in egglog. However, text-based macros are usually a terrible source of bugs.
A more promising direction would be to re-use an existing macro system, such as rust's or racket's.
A #lang egglog would work well
This is related to #232

[segeljakt]

Hmm, I haven't tried Racket but it looks like a good fit. I think there is also a lot of potential in making Egglog more powerful on its own, but maybe this is not important if Egglog is aimed to be a target for code generation.

[oflatt]

Yeah, I think egglog as a target vs a language is up in the air right now

[yihozhang]

We have implemented a macro interface in Rust. See https://github.com/egraphs-good/egglog-experimental/blob/main/src/sugar/for.rs for an example.

Maybe we can implement your macro and call-macro as Rust Macro structs, so that the example in this issue would work. I don't think we will support this macro in the core egglog library though, so I'm closing this for now.