feat(transducers): add DATA-LENS.TRANSDUCERS:INTO, with tests

transducer-protocol.lisp

@@ -16,10 +16,19 @@
                seq)
       acc)))
 
+#+(or)
+(defun document (&rest strings)
+  (serapeum:string-join strings #.(format nil "~2%")))
+
 (defgeneric init (client))
 (defgeneric stepper (client))
 (defgeneric unwrap (client obj)
   (:method (client obj) obj))
+(defgeneric builder-for-input (seq)
+  (:documentation
+   "Take a transducible sequence, return a builder and an init value for that builder.
+
+CONSTRAINT: SEQ should be copied, not modified"))
 
 (defun exit-early (acc)
   (throw 'done acc))
@@ -36,10 +45,29 @@
                                        transducer
                                        (init build)))))))
 
-#+(or)
+(defun into (to xf from)
+  (multiple-value-bind (builder init) (builder-for-input to)
+    (let* ((xf (etypecase xf
+                 (list (apply 'alexandria:compose xf))
+                 ((or function symbol) xf)))
+           (transducer (funcall xf (stepper builder))))
+      (unwrap builder
+              (funcall transducer
+                       (catch 'done
+                         (reduce-generic from
+                                         transducer
+                                         init)))))))
+
+(defmacro defdocumentation (name &body doc-specs)
+  name doc-specs
+  nil)
+
 (defdocumentation transducer-protocol
-    (:function transduce (xf build seq)
-               )
+  (:function transduce (xf builder seq)
+             "Run a transducer XF over sequence SEQ using BUILDER to accumulate results.
+
+Uses the generic function REDUCE-GENERIC so transducers work over lazy
+sequences and hash tables.")
   (:generic-function unwrap (client obj)
                      )
   (:generic-function unwrap (client obj)