[plt-scheme] match patterns for eq?/eqv?/equal? bound values

From: David T. Pierson (dtp at mindstory.com)
Date: Sun May 16 16:05:54 EDT 2010

Hi all,

I am currently using MzScheme 4.2.1

I have been using pattern matching and feel like there is something
missing (or that I am failing to see).  In particular, I am surprised
there are no built-in patterns for matching values that are
eq?/eqv?/equal? to an expected value bound to a variable.  The ? pattern
can obviously be used:

(match v
  ((? (lambda (arg) (eq? arg expected)))

but I was thinking there would be something less verbose, like:

(match v
  ((eq expected)

I defined my own expander(s) to accomplish this, like for eq?:

(define-match-expander eq
  (syntax-rules ()
    ((eq x1)
     (? (lambda (x2) (eq? x1 x2))))))

Obviously for my simple example above a match form is not really needed,
but when nesting patterns this eq pattern becomes quite helpful in
maintaining readability:

(match lst
  ((list (eq foo) (eq bar) rest)

Anyway, I'm wondering if I'm missing an existing facility for doing this
or whether I should simply continue to use my own expanders.



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