I'm starting to use generics, and me being myself, I wrote some macros to make
writing method definitions easier.
But, I'm seeing that #:methods seems to rebind method identifiers in a way that
hygiene interferes with.
I would expect to be allowed to do the following two things (problems
annotated):
(struct exp (label fvs-box)) ;; parent struct for all expressions
(define-generics binds-variables
[free-box binds-variables]
[free binds-variables #:bound [bound]]
#:fallbacks [(define (free e #:bound [bound ∅]) ∅)
(define free-box exp-fvs-box)]
#:fast-defaults ([(compose unbox free-box)
(define (free e #:bound bound) (unbox (free-box e)))])) ;;
problem 1: free-box not in scope
(define-syntax-rule (def-free e gfree bound struct [(pats ...) rhss ...])
(begin
(define/generic gfree free) ;; problem 2: since #:methods rebinds free,
this is not in the scope one would expect with its definition in the
define-generics form.
(define (free e #:bound [bound ∅])
(match e [(struct _ fvs-box pats ...)
(set-box! fvs-box
(let () rhss ...))]))))
(struct var exp (name) #:transparent
#:methods gen:binds-variables
[(def-free e gfree bound var [(x) (if (x . ∈ . bound) ∅ (set x))])])
I have workarounds thanks to stamourv, but they're unpleasant:
Problem 1: define free in fast-defaults as an eta-expansion of a definition
outside the define-generics form that does what you want.
Problem 2: add free as a parameter to def-free, and pass free in at all uses of
def-free.
The first problem seems like more of a programming error than the use of the
wrong tool.
The second problem seems like generic method identifiers should be
syntax-parameters, if they indeed need to be rebound in the rhs of the
#:methods argument.
Are these expectations unreasonable/against the design decisions for generics?
Thanks,
-Ian
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