fixes #541

Signed-off-by: Sean Corfield <sean@corfield.org>

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * 2.6.next in progress
   * Fix [#543](https://github.com/seancorfield/honeysql/issues/543) by supporting both symbols and keywords in named parameters.
+  * Address [#541](https://github.com/seancorfield/honeysql/issues/541) by specifying the expected result of a formatter function passed to `register-clause!` and adding the example from the README to **Extending HoneySQL**.
   * Getting Started updated based on feedback from Los Angeles Clojure meetup walkthrough [#539](https://github.com/seancorfield/honeysql/issues/539).
   * Fix [#538](https://github.com/seancorfield/honeysql/issues/538) by removing `mod` from list of infix operators.
   * Update Clojure version to 1.12.0.

doc/extending-honeysql.md

@@ -50,13 +50,38 @@ The formatter function will be called with:
 * The clause name (always as a keyword),
 * The sequence of arguments provided.
 
+The formatter function should return a vector whose first element is the
+generated SQL string and whose remaining elements (if any) are the parameters
+lifted from the DSL (for which the generated SQL string should contain `?`
+placeholders).
+
 The third argument to `register-clause!` allows you to
 insert your new clause formatter so that clauses are
 formatted in the correct order for your SQL dialect.
 For example, `:select` comes before `:from` which comes
 before `:where`. You can call `clause-order` to see what the
 current ordering of clauses is.
 
+<!-- :test-doc-blocks/skip -->
+```clojure
+;; the formatter will be passed your new clause and the value associated
+;; with that clause in the DSL (which is often a sequence but does not
+;; need to be -- it can be whatever syntax you desire in the DSL):
+(sql/register-clause! :foobar
+                      (fn [clause x]
+                        (let [[sql & params]
+                              (if (ident? x)
+                                (sql/format-expr x)
+                                (sql/format-dsl x))]
+                          (c/into [(str (sql/sql-kw clause) " " sql)] params)))
+                      :from) ; SELECT ... FOOBAR ... FROM ...
+;; example usage:
+(sql/format {:select [:a :b] :foobar :baz})
+=> ["SELECT a, b FOOBAR baz"]
+(sql/format {:select [:a :b] :foobar {:where [:= :id 1]}})
+=> ["SELECT a, b FOOBAR WHERE id = ?" 1]
+```
+
 > Note: if you call `register-clause!` more than once for the same clause, the last call "wins". This allows you to correct an incorrect clause order insertion by simply calling `register-clause!` again with a different third argument.
 
 ## Defining a Helper Function for a New Clause