On 1/31/2019 10:58 AM, hashim muqtadir wrote:
The test in my following code raises an error saying that when I call
select/f, the result doesn't satisfy `sql-statement?`.
I've no idea why, the only vague idea I have is that it may have
something to do with the fact that it was essentially eval'd in a
separate namespace.
Any ideas?
#lang racket
(require db)
(define sql-namespace (make-base-empty-namespace))
(parameterize ([current-namespace sql-namespace])
(namespace-require 'sql sql-namespace))
(struct Select-spec
(tbl ;; symbol
col-exprs ;; listof symbols
limit ;; number
order-by)) ;; listof select symbols
(define (select/f sel-spec)
(eval
`(select ,@(Select-spec-col-exprs sel-spec)
#:from ,(Select-spec-tbl sel-spec)
,@(let ([lim (Select-spec-limit sel-spec)]
[order (Select-spec-order-by sel-spec)])
(if lim
`(#:limit ,lim #:order-by ,@order)
'())))
sql-namespace))
(module+ test
(require sql)
(require rackunit)
(check-equal?
(sql-statement->string
(select/f
(Select-spec
'd
'(a b c)
10
'(a b))))
(sql-statement->string
(select a b c
#:from d #:order-by a b #:limit 10))))
(provide select/f
Select-spec)
My main reason for doing this was that the statement-producing macros
provided by the sql package were, well, macros, so I couldn't pass in
a list of columns and get a statement. What I want to do is have a
struct like the one I made in this snippet, and use functions to
include/exclude columns, before finally passing it off to a function
that produces the statement.
I agree with John that you probably should not be using eval here. Eval
almost always is the wrong answer.
But as to why your code is not working: there is a problem with structs
and namespaces - I don't recall the technical specifics, but the gist is
that simply copying a struct into a different namespace doesn't work -
it results in a new, auto-created struct type - which isn't what you
want. If order to use a struct across namespaces, the module that
defines the struct must be required into each namespace that uses the
struct.
George
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