Following up on How to make a record from a sequence of values, how can you write a defrecord constructor call and assign the fields from a Map, leaving un-named fields nil?
(defrecord MyRecord [f1 f2 f3])
(assign-from-map MyRecord {:f1 "Huey" :f2 "Dewey"}) ; returns a new MyRecord
I imagine a macro could be written to do this.
You can simply merge the map into a record initialised with nils:
(merge (MyRecord. nil nil nil) {:f1 "Huey" :f2 "Dewey"})
Note that records are capable of holding values stored under extra keys in a map-like fashion.
The list of a record's fields can be obtained using reflection:
(defn static? [field]
(java.lang.reflect.Modifier/isStatic
(.getModifiers field)))
(defn get-record-field-names [record]
(->> record
.getDeclaredFields
(remove static?)
(map #(.getName %))
(remove #{"__meta" "__extmap"})))
The latter function returns a seq of strings:
user> (get-record-field-names MyRecord)
("f1" "f2" "f3")
__meta and __extmap are the fields used by Clojure records to hold metadata and to support the map functionality, respectively.
You could write something like
(defmacro empty-record [record]
(let [klass (Class/forName (name record))
field-count (count (get-record-field-names klass))]
`(new ~klass ~@(repeat field-count nil))))
and use it to create empty instances of record classes like so:
user> (empty-record user.MyRecord)
#:user.MyRecord{:f1 nil, :f2 nil, :f3 nil}
The fully qualified name is essential here. It's going to work as long as the record class has been declared by the time any empty-record forms referring to it are compiled.
If empty-record was written as a function instead, one could have it expect an actual class as an argument (avoiding the "fully qualified" problem -- you could name your class in whichever way is convenient in a given context), though at the cost of doing the reflection at runtime.