feat: clarify default parameters for passes

[BrayanCaro]

I think we can use default values for extra formal parameters in the define-pass form:

(define-pass name : lang-specifier (formal ...) -> lang-specifier (extra-return-val ...)
  definitions-clause
  processor ...
  body-expr)

And the following code works:

(define-language L
  (entry Expr)
  (terminals
   (number (n)))
  (Expr (e)
        n
        (someop e)))
(define-parser parse-L L)

(define-pass as-bin : L (ir global-ctx) -> L ()
  ( as-binary : Expr (ir [ctx global-ctx]) -> Expr ()
              [,n (modulo n 2)]
              [(someop ,[e? 'val-b -> e]) `(someop ,e)]))
; usage:
; (as-bin (parse-L '(someop 4)) 'val-default)
; or
; ((compose (curryr as-bin 'val-default) parse-L) '(someop 4))

But only the processor as-binary can have the default value, and I think It would be cool to be able to also write default values as the define statement does:

;                           ⬐---------here---------⬎
(define-pass as-bin : L (ir [global-ctx 'val-default]) -> L ()
  ( as-binary : Expr (ir [ctx global-ctx]) -> Expr ()
              [,n (modulo n 2)]
              [(someop ,[e? 'val-b -> e]) `(someop ,e)]))

; suggested usage:
; (as-bin (parse-L '(someop 4)))
; or
; ((compose as-bin parse-L) '(someop 4))

Note

The documentation does not say anything about default parameters:

formal is an identifier representing a formal argument.