[racket-dev] Generics scoping issues

From: Carl Eastlund (cce at ccs.neu.edu)
Date: Fri Aug 16 16:50:53 EDT 2013

Problem 1 -- you have to use define/generic if you want to use the generic
version of something in the context of a set of specific method
implementations.  That's by design.

Problem 2 -- what error message or unexpected behavior are you getting?
That should work, it sounds like a bug in define/generic if it's not

Carl Eastlund

On Fri, Aug 16, 2013 at 4:36 PM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:

> 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
> _________________________
>   Racket Developers list:
>   http://lists.racket-lang.org/dev
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