I like to build DSLs when I code, so that at a high level, much of the
logic is readable and consistent. I'm trying to create a macro that
will verify the uniqueness of a record based on various fields in the
record. For example, if I have a function that looks up a record by
its name:
(defn by-name [name] (...some code to look up the record by
name...))
In its create routine, I want to call a macro "unique?" and have the
record check that the name doesn't already exist:
(defn create
[name]
(unique? :name name "The name \"%s\" already exists.")
....no exception, we can create the record...
)
That "unique?" macro would convert :name into a call to the "by-name"
function, pass in the value, check for a result, and if it already
exists, throw an exception with the given message.
I'm having a heck of a time converting that :name to the "by-name"
reference, and I'm trying to avoid eval because I understand it's a
performance hit. Is there a way to accomplish this in the "unique?"
macro?
Here's my first attempt (obviously it doesn't work, but might help
clarify what I'm getting at):
(defmacro unique?
"Is the given value unique to the record?"
[name value message]
`(if-not (str/blank? ~value)
(let [finder# (symbol (str "by-" (name ~name)))
existing# (finder# ~value)]
(if existing# (duplicate existing# ~message ~value)))))
(The "duplicate" function raises the exception.)
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