<div dir="ltr"><div>OK.</div><div><br></div><div><span style="font-family:monospace">procedure foo is<br></span></div><div><span style="font-family:monospace"> x : Integer := 42;</span></div><div><span style="font-family:monospace">begin</span></div><div><span style="font-family:monospace"> ...<br></span></div><div><span style="font-family:monospace">end</span></div><div><br></div><div><br></div><div><div>Now <span style="font-family:monospace">foo</span> depends on the hardwired '42' (as is should, one may argue :) ). And we are not even talking about "classes" or Lisp here.<br><br></div><div>Have I boiled down to the essentials? How do you do the rolling eyes emoticon?</div><div><br></div><div>All the best</div><div><br></div><div>Marco</div><div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Feb 6, 2021 at 10:50 PM Pascal Bourguignon <<a href="mailto:pjb@informatimago.com">pjb@informatimago.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Le 06/02/2021 à 22:37, Manfred Bergmann a écrit :<br>
> You have a class. This class uses some other class.<br>
> But by using or creating an instance of this other class directly you create a dependency on something concrete.<br>
> That’s not what you want, because you might want to replace this with something else if required. For example with a mock or fake implementation in a test.<br>
> ‚Dependency injection‘ allows you to declare this dependency with just an interface/protocol and have some other facility (the dependency injection framework) ‚inject‘ a concrete object at run-time.<br>
> A similar thing could certainly be done by just using a constructor parameter (strategy pattern).<br>
> But I think the important part here is the dependency on just an interface and not on a concrete implementation. For flexibility.<br>
<br>
<br>
With some code:<br>
<br>
;;;------------------------------------------------------------<br>
<br>
(defclass used ()<br>
())<br>
<br>
(defmethod used-stuff ((self used))<br>
'stuff)<br>
<br>
;;; ---<br>
<br>
(defclass user ()<br>
((used :reader used)))<br>
<br>
(defmethod initialize-instance :after ((self user) &key &allow-other-keys)<br>
(setf (slot-value self 'used) (make-instance 'used #|OOPS, <br>
Dependency!|#)))<br>
<br>
(defmethod user-stuff ((self user))<br>
;; Not a real dependency on the used class,<br>
;; it's a dependency on the used-stuff generic function (interface).<br>
(used-stuff (used self)))<br>
<br>
;;; ---<br>
<br>
(defclass client ()<br>
())<br>
<br>
(defmethod create-user ((self client))<br>
;; The class client depends directly on the user class,<br>
;; and indirectly on the used class.<br>
(make-instance 'user))<br>
<br>
<br>
;;;------------------------------------------------------------<br>
<br>
(defclass used ()<br>
())<br>
<br>
(defmethod used-stuff ((self used))<br>
'stuff)<br>
<br>
;;; ---<br>
<br>
(defclass user ()<br>
((used :initarg :used :reader used)))<br>
<br>
;; The user class has no more any dependency on the used class.<br>
<br>
(defmethod user-stuff ((self user))<br>
;; Not a real dependency on the used class,<br>
;; it's a dependency on the used-stuff generic function (interface).<br>
(used-stuff (used self)))<br>
<br>
;;; ---<br>
<br>
(defclass client ()<br>
())<br>
<br>
(defmethod create-user ((self client))<br>
;; The class client depends explicitely on the user and used classes.<br>
;; But now, the class user doesn't depend directly on the used class;<br>
;; this dependency is injected by the client into the user classe:<br>
(make-instance 'user :used (make-instance 'used)))<br>
<br>
<br>
;;;------------------------------------------------------------<br>
<br>
;; Notably if the client wants the user to use another used class:<br>
<br>
(defclass variant-used (used)<br>
())<br>
(defmethod used-stuff ((self variant-used))<br>
'variant-stuff)<br>
<br>
(defmethod create-user ((self client))<br>
;; only the client needs to be changed; the user class won't know<br>
;; the difference:<br>
(make-instance 'user :used (make-instance 'variant-used)))<br>
<br>
<br>
-- <br>
__Pascal Bourguignon__<br>
<br>
</blockquote></div><br clear="all"><br>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">Marco Antoniotti, Associate Professor</span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px;text-decoration:none"> </span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px;text-decoration:none"> </span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">tel.</span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;text-decoration:none;display:inline;float:none;white-space:pre"> </span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">+39 - 02 64 48 79 01</span><br style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">DISCo, Università Milano Bicocca U14 2043</span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px;text-decoration:none"> </span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px;text-decoration:none"> </span><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px;text-decoration:none"></span><a href="http://bimib.disco.unimib.it/" style="font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">http://bimib.disco.unimib.it</a><br style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">Viale Sarca 336</span><br style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="color:rgb(0,0,0);font-family:Helvetica;font-size:14px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;display:inline;float:none">I-20126 Milan (MI) ITALY</span><br></div></div></div></div>