Hi everyone,
I have started playing with generics and quickly got myself in trouble.
Consider the following two modules:
-- foo.rkt ------------------------------------------------
#lang racket
(provide (all-defined-out))
(require racket/generic)
(define-generics bar
[baz bar])
-----------------------------------------------------------
-- foo-bar.rkt --------------------------------------------
#lang racket
(provide (all-defined-out))
(require (prefix-in foo: "foo.rkt"))
(define (baz x)
(list x x))
(struct foo-bar (quux)
#:methods foo:gen:bar
[(define (foo:baz x) (baz (foo-bar-quux x)))])
-----------------------------------------------------------
Then, in a REPL:
(require (prefix-in foo: "foo.rkt")
"foo-bar.rkt")
(foo:baz (foo-bar 42))
This yields the error message:
baz: not implemented for #<foo-bar>
By trial and error I figured out that in the method definition I need
to use the method name as written in define-generics, in spite of the
(require (prefix-in ...)). No problem, it seems:
-----------------------------------------------------------
(struct foo-bar (quux)
#:methods foo:gen:bar
[(define (baz x) (baz (foo-bar-quux x)))])
-----------------------------------------------------------
But now the call to baz inside the method becomes a recursive call to
the method, rather than a call to the function baz defined in the
surrounding module. In fact, that's why I used (prefix-in ...) in the
first place.
I tried to get around this with a local binding:
-----------------------------------------------------------
(let ([temp baz])
(struct foo-bar (quux)
#:methods foo:gen:bar
[(define (baz x) (temp (foo-bar-quux x)))]))
-----------------------------------------------------------
for which the punishment is:
begin (possibly implicit): no expression after a sequence of internal
definitions
Any ideas?
Konrad.
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