Re: problem 1 - ah yes I see.
Problem 2:
In the body of def-free:
generic-problems.rkt:16:26: define/generic: free is not a method of generic
interfaces gen:binds-variables
at: free
in: (define/generic gfree free)
This compiles if I additionally supply def-free with free, but it doesn't run:
free-box: undefined;
cannot reference an identifier before its definition
in module: "/home/ianj/projects/oaam/code/generic-problems.rkt"
context...:
/home/ianj/projects/oaam/code/generic-problems.rkt: [running body]
Furthermore, if I abstract over the struct form so that #:methods
gen:binds-variables is part of the macro expansion, even passing free to
def-free won't work anymore:
(define-syntax-rule (astruct name (fields ...) (methods ...))
(struct name exp (fields ...) #:transparent #:methods gen:binds-variables
[methods ...]))
(astruct avar (name)
[(def-free e gfree free bound avar [(x) (if (x . ∈ . bound) ∅ (set
x))])])
generic-problems.rkt:31:27: define/generic: free is not a method of generic
interfaces gen:binds-variables
at: free
in: (define/generic gfree free)
----- Original Message -----
From: "Carl Eastlund" <c...@ccs.neu.edu>
To: "J. Ian Johnson" <i...@ccs.neu.edu>
Cc: "dev" <dev@racket-lang.org>
Sent: Friday, August 16, 2013 4:50:53 PM GMT -05:00 US/Canada Eastern
Subject: Re: [racket-dev] Generics scoping issues
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 working.
Carl Eastlund
On Fri, Aug 16, 2013 at 4:36 PM, J. Ian Johnson < i...@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
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