How to use pprint-newline properly?

Question

I am trying to print a sequence such that neither the whole sequence is printed on one line, nor is each element of the sequence printed on its own line. E.g.

[10 11 12 13 14 15 16 17 18 19
 20 21 22 23 24 25 26 27 28 29]

I found pprint-newline in the documentation which indicates that allows me to determine how the newline gets printed. Unfortunately, I cannot find any examples on how it is to be used in conjunction with pprint, and the doc string doesn't to offer much insight:

-------------------------
clojure.pprint/pprint-newline
([kind])
  Print a conditional newline to a pretty printing stream. kind specifies if the 
newline is :linear, :miser, :fill, or :mandatory. 

This function is intended for use when writing custom dispatch functions.

Output is sent to *out* which must be a pretty printing writer.

pprint specifies an optional second argument for the writer, which is by default set to *out*. However, I am not sure how to 'send' pprint-writer to *out* in this case, e.g. something like the example below doesn't appear to work

(clojure.pprint/pprint [1 2 3 4] (*out* (clojure.pprint/pprint-newline :miser)))

score 3 · Accepted Answer · answered Oct 18 '13 at 16:38

3

While Guillermo explained how to change the dispatch for pretty-printing in general, if all you want to do is printing one collection differently, that's possible, too.

For example, using cl-format (after (use '[clojure.pprint :as pp)):

(binding [pp/*print-pretty* true
          pp/*print-miser-width* nil
          pp/*print-right-margin* 10]
  (pp/cl-format true "~<[~;~@{~a~^ ~:_~}~;]~:>~%" '[foo bar baz quux]))

Set *print-right-margin* as you wish.

You don't have to use format for this. The format directives can be translated to their respective pretty-printer functions, if you want. Explanation of the format string: ~< and ~:> establish a logical block. Inside the block, there are three sections separated by ~;. The first and last section are your prefix and suffix, while the elements are printed in the middle section, using ~@{ and ~}. For each element, the element is printed using ~a, followed by a space, if needed, and a conditional fill-style newline.

(In CL, the format string could be simplified to "~<[~;~@{~a~^ ~}~;]~:@>~%", but that doesn't seem to work in Clojure 1.5.)

answered Oct 18 '13 at 16:38

danlei

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Thanks for the solution - it works perfectly. However, can you please tell me what you mean by the following: "The format directives can be translated to their respective pretty-printer functions, if you want". Would this entail writing a dispatch function as Guillermo indicated below? Perhaps that would be a better alternative to the arcane CL style `format` string... – dtg Oct 19 '13 at 05:51
There is a correspondence between the format directives and the functions in Guillermo's answer: `~< … ~:>` is `pprint-logical-block`, `~@{ … ~}` is `print-length-loop`, etc. Note that I'm using `~:_`, corresponding to `(pprint-newline :fill)`, not `:linear`. You *should* be able to use these functions directly for printing, not just for dispatching, but you'll have to set up everything properly, i.e. use `get-pretty-writer` with `*out*` etc. Just `binding` it didn't work for me, though. I had some success with `set!`, but that feels hackish, so check the documentation (or source) for that. – danlei Oct 19 '13 at 13:17
Thanks again. I might have to break open the Common Lisp hyperspec and read up on the CL formatting... – dtg Oct 19 '13 at 18:09
1

You're welcome. Yes, reading about the CL pretty-printer would certainly help. But while the Clojure pretty-printer is strongly influenced by it (XP), and the general concepts should be the same, there will be differences under the hood. (Like setting up `*out*` correctly, as mentioned above.) If all you want to do is print one collection, the `cl-format` solution should suffice. Sure, it looks rather arcane, but on the other hand, it's a powerful, compact DSL for formatted output. For anything more complex (e.g. a custom data format or language), I'd suggest creating a custom dispatch table. – danlei Oct 19 '13 at 19:36

score 1 · Answer 2 · answered Oct 18 '13 at 15:42

As the help says, the function is intended for use for custom dispatch functions.

In order to change the behavior of the pprint for sequences you need to provide a new dispatch function for clojure.lang.ISeq.

The current dispatch function for sequences you can find in clojure/pprint/dispatch.clj

(use-method simple-dispatch clojure.lang.ISeq pprint-list) 
...
(defn- pprint-simple-list [alis]
    (pprint-logical-block :prefix "(" :suffix ")"
      (print-length-loop [alis (seq alis)]
        (when alis
      (write-out (first alis))
      (when (next alis)
        (.write ^java.io.Writer *out* " ")
        (pprint-newline :linear)
        (recur (next alis)))))))

Since printing is dispatched according to data type overriding seems to be the way to go.

See the source code for ideas.

Thank you. This seems like the way to go, ultimately. But for now, danlei's solution satisfies my requirements. — dtg, Oct 19 '13 at 18:08

How to use pprint-newline properly?

2 Answers2