feat: add tap

lens.lisp

@@ -179,6 +179,11 @@
   (lambda (seq)
     (map 'list selector seq)))
 
+(defun tap (cb)
+  (lambda (it)
+    (prog1 it
+      (funcall cb it))))
+
 (defun slice (start &optional end)
   (lambda (it)
     (subseq it start end)))

package.lisp

@@ -21,7 +21,8 @@
            #:transform-elt #:denest #:op #:defalias #:<> #:<>1 #:== #:•
            #:∘ #:suffixp #:functionalize #:inc #:group-by #:keys
            #:conj #:disj #:delay #:of-type #:transform #:calling*
-           #:calling))
+           #:calling
+           #:tap))
 
 (defpackage :data-lens.transducers.internals
   (:use :cl)