`eval` invokes the Clojure compiler, which transforms data structures into
Java bytecode. The Clojure compiler understands Clojure data structures
like lists, vectors, and symbols, plus a few Java types like String and
numbers. It doesn't know what to do with a java.util.Date. "Can't embed
object in code" is the compiler telling you "I don't know what to do with
this."
`print-dup` is an internal multimethod used by the Clojure compiler to
transform complex objects, like sorted sets, into data structures
representing the Clojure code to construct them. For example,
user=> (print-dup (sorted-set) *out*)
#=(clojure.lang.PersistentTreeSet/create [])
The #= is a Clojure reader macro that means "evaluate the following code."
The error message "maybe print-dup not defined" is suggesting that, if you
really did mean to stick a java.util.Date in your compiled code, you have to
provide your own implementation of print-dup. Specifically, you need a
method of print-dup that takes a java.util.Date and returns a data structure
representing the Clojure code that *creates* a java.util.Date. For example:
user=> (defmethod print-dup java.util.Date [d stream]
(.write stream "#=(java.util.Date. ")
(.write stream (str (.getTime d)))
(.write stream ")"))
user=> (eval [(java.util.Date.)])
[#<Date Fri Mar 11 23:47:13 EST 2011>]
However, this is probably not what you really meant to do. More likely, you
have a macro or `eval` somewhere that is using code that is incorrectly
quoted.
-Stuart Sierra
clojure.com
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