On Sep 22, 2:23 am, Jung Ko <koj...@gmail.com> wrote:
> Hi,
>
> Can someone teach me how I can override the "toString" method for a struct ?
>
> Here's a code snippet that shows what I'm trying to do:
>
> user> (defstruct bookinfo :book :filename)
> user> (struct bookinfo "hello" "world") => {:book "hello", :filename "world"}
>
> I would like to override the "toString" method on a struct so that the
> following behavior is true:
>
> user> (struct bookinfo "hello" "world") => "hello"
>
> In Common Lisp, I would simply define a method on (print-object). What
> would be the equivalent thing in Clojure?
As Mike said, it's done by adding a method to print-method, but you
will need metadata... "structs" in clojure are nothing but an
optimisation of maps... That is, in all situations structmaps are (at
least, should be) interchangeable with a regular persistent map. They
do not create their own type or anything, so you can't override their
toString method directly.
However, since print-method uses :type metadata, you can do the
following:
;; using ::foo to get namespace-qualified keywords is good for type
tags, because it avoids collisions
;; typed-book could be a struct-map, or in fact any implementation of
Map that also supports metadata
(def typed-book (with-meta {:book "somebook"} {:type ::my-book})) ;
the latter map is the metadata
(defmethod print-method ::my-book [thebook writer]
(print-method (:book thebook) writer)) ; falls back to the string
writer
(print typed-book) ; prints 'somebook'
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