I'm finding parameters confusing and was hoping someone could explain them
for me. The following is a simplified version of our production code; I'm
expecting it to fail but it does not. What am I not understanding?
The sequence goes:
*) db.conf creates and provides some parameters (db username, db password,
db name, db port) and a function for creating database connections using
those parameters
*) dbstuff.rkt requires db.conf, re-provides the parameters from db.conf,
and also provides a wrapper around the db connector from db.conf
*) bar.rkt parses the command line, uses that command line data to set the
parameters that were imported from db.conf by way of dbstuff.rkt and makes
a DB call
*) I run bar.rkt from the command line with an invalid username for the
database, expecting it to fail to connect, but it works fine???
My first guess was that since I was using the parameters as default values
perhaps they were being evaluated at compile time instead of runtime. I
tried changing the default value to #f and then doing (or u (db-user)) in
the body of the function, but that had no effect.
I am utterly baffled at this point. Any help would be much appreciated.
################################# file: "db.conf"
#lang racket
(require db)
(define db-user (make-parameter "postgres"))
(define db-name (make-parameter "test-db"))
(define db-password (make-parameter "mellon"))
(define db-port-num (make-parameter 5433))
(define (do-connect #:u [u (db-user)]
#:d [d (db-name)]
#:p [p (db-password)]
#:prt [prt (db-port-num)])
(println (~a "In db.conf db-user: " (db-user)))
(println (~a "db-name: " (db-name)))
(println (~a "db-password: " (db-password)))
(println (~a "db-port: " (db-port-num)))
(postgresql-connect #:user u
#:database d
#:password p
#:port prt
))
(define dbh (virtual-connection
(connection-pool
do-connect)))
(define (make-connect)
dbh)
(provide (all-defined-out))
################################# file: "dbstuff.rkt"
#lang racket
(require db)
(require "db.conf")
(define/contract (dbh)
(-> connection?)
(make-connect))
(provide (all-defined-out)
(all-from-out db)
(all-from-out "db.conf"))
################################# file: "bar.rkt"
#lang racket
(require "dbstuff.rkt")
(command-line
#:program "db demo"
#:once-each
[("--db-user") dbu "Username for database handles"
(db-user dbu)]
)
(println (~a "In bar: db user: " (db-user)))
(query-value (dbh) "select 'this should not have worked'")
################################# command line:
$ racket bar.rkt --db-user invalid_user
"In bar: db user: invalid_user"
"In db.conf db-user: postgres"
"db-name: test-db"
"db-password: mellon"
"db-port: 5433"
"this should not have worked"
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