On 11 May 2014 15:18, Hussein B. <hubaghd...@gmail.com> wrote:
> Hi,
>
> I'm trying to learn how to make DSL in Clojure and Korma project is a
> really good place to learn from. I'm trying this very simple stuff (inspired
> by Korma, not Korma code):
[code snipped]
> But when I'm trying in the REPL:
>
> (select
> (where
> (and (= :version 1.6) (= :name "Clojure"))
>
> I'm getting:
>
> ClassCastException java.lang.Character cannot be cast to
> clojure.lang.IPersistentCollection clojure.core/conj (core.clj:83)
>
> It is really hard to me to know what is going wrong. What is confusing me,
> if I'm trying the where macro alone, it is working as excepted but when
> integrating it with select macro, I got the exception.
Generally, when macros are causing problems, it's good to start
debugging with macroexpand to see what's actually getting executed.
> (macroexpand '(select
(where
(and (= :version 1.6) (= :name "Clojure")))))
; (let* [query-map__696__auto__ (clojure.core/-> (#'dsltmp/select*)
(where (and (= :version 1.6) (= :name "Clojure"))))]
query-map__696__auto__)
I then tried macroexpand on the where clause that that would generate:
> (macroexpand '(where select* (and (= :version 1.6) (= :name "Clojure"))))
; (let* [q__703__auto__ select*] (#'dsltmp/where* q__703__auto__
(dsltmp/clauses->vec (:and (:eq :version 1.6) (:eq :name
"Clojure")))))
So the argument getting passed into clauses->vec is (:and (:eq
:version 1.6) (:eq :name "Clojure")) - but it's not a macro, so this
is evaluated:
> (:and (:eq :version 1.6) (:eq :name "Clojure"))
; "Clojure"
(If it's not clear what's going on here, remember that keywords try to
look themselves up in their first argument, and default to their
second argument.)
So 'clauses' is "Clojure", '(rest clauses)' is '(\l \o \j \u \r \e)',
and condition->map is invoked with the argument \l, which triggers the
error you saw:
> (condition->map \l)
; ClassCastException java.lang.Character cannot be cast to
clojure.lang.IPersistentCollection clojure.core/conj (core.clj:83)
Since the problem is that clauses->vec evaluates its argument,
defining it as a macro instead of a function should fix it.
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