<div dir="ltr"><div><div>*wrap, not rap. In my previous mail.<br><br></div>Also, there is already a library that does list operations in a safe way, though they have used monads.  Give a look at the source code and see if it helps.<br><br><a href="https://hackage.haskell.org/package/listsafe-0.1.0.0/docs/Data-List-Safe.html">https://hackage.haskell.org/package/listsafe-0.1.0.0/docs/Data-List-Safe.html</a><br></div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 13, 2015 at 3:19 PM, akash g <span dir="ltr"><<a href="mailto:akaberto@gmail.com" target="_blank">akaberto@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div>Ah, but it doesn't.  Since this is a functional language, we have a pointer to it.  In fact, all it does is rap it around in the Maybe type.<br><br></div>Also, my version doesn't do what you want.  One way to do that would be what you have.  Or something like this.<br><br></div>The following turns the partial function in the Prelude to one that is safe<br><br>==========<br>import Data.List<span class=""><br><br>init' :: [a] -> Maybe [a]<br></span>init' [] = Nothing          -- Base case<br>init' xs = Just $ tail xs   -- Inductive case <br>==========<br><br><br><div><div><div><br><br><br><br></div></div></div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 13, 2015 at 2:47 PM, Roelof Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
  
    
  
  <div bgcolor="#FFFFFF" text="#000000">
    <div>Hello, <br>
      <br>
      What my intention was to make a safe version of the init function
      found in Data.list so it procudes a list without the last item. <br>
      At first look your function procudes the whole list again.<br>
      <br>
      Roelof<br>
      <br>
      <br>
      <br>
      akash g schreef op 13-5-2015 om 10:57:<br>
    </div><div><div>
    <blockquote type="cite">
      <div dir="ltr">
        <div>
          <div>
            <div>
              <div>Why not just?<br>
                <br>
                ==================<br>
              </div>
              init' [] = Nothing<br>
            </div>
            init' xs = Just xs<br>
            ==================<br>
            <br>
          </div>
          <div>Meets your type sig and is also has a time complexity of
            O(1) instead of O(n) which will be the time complexity in
            the solution involving fromMaybe.  Maybe I'm missing
            something.<br>
          </div>
          <div><br>
          </div>
          Perhaps it'd help us help you if you'd be a bit more clear on
          what you want to achieve. </div>
      </div>
      <div class="gmail_extra"><br>
        <div class="gmail_quote">On Tue, May 12, 2015 at 9:01 PM, Sumit
          Sahrawat, Maths & Computing, IIT (BHU) <span dir="ltr"><<a href="mailto:sumit.sahrawat.apm13@iitbhu.ac.in" target="_blank">sumit.sahrawat.apm13@iitbhu.ac.in</a>></span>
          wrote:<br>
          <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
            <div dir="ltr">
              <div class="gmail_extra">
                <div class="gmail_quote"><span>On 12 May 2015
                    at 20:14, Roelof Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span>
                    wrote:<br>
                    <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
                      <div bgcolor="#FFFFFF" text="#000000">
                        <div>Thanks, <br>
                          <br>
                          The right solution was this : <br>
                          <br>
                          init' (x:xs) = Just (x:fromMaybe xs (init'
                          xs))<br>
                          <br>
                          if I understand it right x:fromMaybe xs takes
                          care that from xs the just or Nothing is
                          removed ?<br>
                          <br>
                        </div>
                      </div>
                    </blockquote>
                    <div><br>
                    </div>
                  </span>Trying it out in ghci,
                  <div><br>
                  </div>
                  <div>fromMaybe [1,2] Nothing == [1,2]</div>
                  <div>fromMaybe [1,2] Just [3] == [3]</div>
                  <div><br>
                  </div>
                  <div>It seems like that should indeed work.</div>
                  <div>
                    <div>
                      <div><br>
                      </div>
                      <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
                        <div bgcolor="#FFFFFF" text="#000000">
                          <div> Roelof<br>
                            <br>
                            <br>
                            <br>
                            Alexey Shmalko schreef op 12-5-2015 om
                            16:33:<br>
                          </div>
                          <div>
                            <div>
                              <blockquote type="cite">
                                <div dir="ltr">Try fromMaybe [1].
                                  Examples<br>
                                  <br>
                                  <div>Prelude Data.Maybe> fromMaybe
                                    [] (Just [1,2,3])</div>
                                  <div>[1,2,3]</div>
                                  <div>Prelude Data.Maybe> fromMaybe
                                    [1,2,3] Nothing</div>
                                  <div>[1,2,3]</div>
                                  <div><br>
                                  </div>
                                  <div>[1]: <a href="https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Maybe.html#v:fromMaybe" target="_blank">https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Maybe.html#v:fromMaybe</a><br>
                                  </div>
                                </div>
                                <br>
                                <div class="gmail_quote">On Tue, May 12,
                                  2015 at 5:28 PM Roelof Wobben <<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>>
                                  wrote:<br>
                                  <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
                                    <div bgcolor="#FFFFFF" text="#000000">
                                      <div>Oke, <br>
                                        <br>
                                        And how do I do this.  Haskell
                                        is a difficult one to learn,<br>
                                        <br>
                                        Roelof<br>
                                        <br>
                                        <br>
                                        <br>
                                        Alexey Shmalko schreef op
                                        12-5-2015 om 16:24:<br>
                                      </div>
                                    </div>
                                    <div bgcolor="#FFFFFF" text="#000000">
                                      <blockquote type="cite">
                                        <div dir="ltr">Before cons'ing
                                          the result of init', you
                                          should check whether it's Just
                                          or Nothing. What you're doing
                                          now is something along the
                                          line with 5 : Just 3 -- this
                                          won't typecheck.<br>
                                        </div>
                                        <br>
                                        <div class="gmail_quote">On Tue,
                                          May 12, 2015 at 5:22 PM Roelof
                                          Wobben <<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>>

                                          wrote:<br>
                                          <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
                                            <div bgcolor="#FFFFFF" text="#000000">
                                              <div>Brandon Allbery
                                                schreef op 12-5-2015 om
                                                16:16:<br>
                                              </div>
                                            </div>
                                            <div bgcolor="#FFFFFF" text="#000000">
                                              <blockquote type="cite">
                                                <div dir="ltr">
                                                  <div class="gmail_extra">
                                                    <div class="gmail_quote">On
                                                      Tue, May 12, 2015
                                                      at 10:11 AM,
                                                      Roelof Wobben <span dir="ltr"><<a href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>></span>
                                                      wrote:<br>
                                                      <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
                                                        <div bgcolor="#FFFFFF" text="#000000">
                                                          <div>I do not
                                                          understand
                                                          what you are
                                                          saying to me.<br>
                                                          <br>
                                                          I know that
                                                          init produces
                                                          a Maybe [a] .
                                                          That is why I
                                                          did put a Just
                                                          before it.<br>
                                                          </div>
                                                        </div>
                                                      </blockquote>
                                                      <div><br>
                                                      </div>
                                                      <div>You are
                                                        invoking it
                                                        again though,
                                                        and using its
                                                        result as if it
                                                        produces [a]
                                                        instead of Maybe
                                                        [a].</div>
                                                    </div>
                                                    <div><br>
                                                    </div>
                                                    -- <br>
                                                    <div>
                                                      <div dir="ltr">
                                                        <div>brandon s
                                                          allbery kf8nh
                                                                       
                                                                       
                                                            sine nomine
                                                          associates</div>
                                                        <div><a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a>
                                                                       
                                                                       
                                                               <a href="mailto:ballbery@sinenomine.net" target="_blank">ballbery@sinenomine.net</a></div>
                                                        <div>unix,
                                                          openafs,
                                                          kerberos,
                                                          infrastructure,
                                                          xmonad        <a href="http://sinenomine.net" target="_blank">http://sinenomine.net</a></div>
                                                      </div>
                                                    </div>
                                                  </div>
                                                </div>
                                                <br>
                                                <fieldset></fieldset>
                                                <br>
                                                <pre>_______________________________________________
Beginners mailing list
<a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a>
</pre>
                                              </blockquote>
                                              <br>
                                            </div>
                                            <div bgcolor="#FFFFFF" text="#000000"> What is
                                              then the right way to do.
                                              I still do not understand
                                              what you are trying to
                                              make clear to me.</div>
                                            <div bgcolor="#FFFFFF" text="#000000"><br>
                                              <br>
                                              Roelof</div>
                                            <div bgcolor="#FFFFFF" text="#000000"><br>
                                              <br>
                                              <br>
                                              <br>
                                              <hr style="border:none;color:rgb(144,144,144);min-height:1px;width:99%;background-color:rgb(176,176,176)">
                                              <table style="border-collapse:collapse;border:none">
                                                <tbody>
                                                  <tr>
                                                    <td style="border:none;padding:0px 15px 0px 8px"> <a href="http://www.avast.com/" target="_blank"> <img src="http://static.avast.com/emails/avast-mail-stamp.png" alt="Avast
                                                          logo" border="0"> </a>
                                                    </td>
                                                    <td>
                                                      <p style="color:rgb(61,77,90);font-family:Calibri,Verdana,Arial,Helvetica;font-size:12pt">
                                                        Dit
                                                        e-mailbericht is
                                                        gecontroleerd op
                                                        virussen met
                                                        Avast
                                                        antivirussoftware.
                                                        <br>
                                                        <a href="http://www.avast.com/" target="_blank">www.avast.com</a> </p>
                                                    </td>
                                                  </tr>
                                                </tbody>
                                              </table>
                                              <br>
                                            </div>
_______________________________________________<br>
                                            Beginners mailing list<br>
                                            <a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a><br>
                                            <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
                                          </blockquote>
                                        </div>
                                        <br>
                                        <fieldset></fieldset>
                                        <br>
                                        <pre>_______________________________________________
Beginners mailing list
<a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a>
</pre>
                                      </blockquote>
                                      <br>
                                      <br>
                                      <br>
                                      <hr style="border:none;color:rgb(144,144,144);min-height:1px;width:99%;background-color:rgb(176,176,176)">
                                      <table style="border-collapse:collapse;border:none">
                                        <tbody>
                                          <tr>
                                            <td style="border:none;padding:0px 15px 0px 8px"> <a href="http://www.avast.com/" target="_blank"> <img src="http://static.avast.com/emails/avast-mail-stamp.png" alt="Avast
                                                  logo" border="0"> </a>
                                            </td>
                                            <td>
                                              <p style="color:rgb(61,77,90);font-family:Calibri,Verdana,Arial,Helvetica;font-size:12pt">
                                                Dit e-mailbericht is
                                                gecontroleerd op
                                                virussen met Avast
                                                antivirussoftware. <br>
                                                <a href="http://www.avast.com/" target="_blank">www.avast.com</a> </p>
                                            </td>
                                          </tr>
                                        </tbody>
                                      </table>
                                      <br>
                                    </div>
_______________________________________________<br>
                                    Beginners mailing list<br>
                                    <a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a><br>
                                    <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
                                  </blockquote>
                                </div>
                                <br>
                                <fieldset></fieldset>
                                <br>
                                <pre>_______________________________________________
Beginners mailing list
<a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a>
</pre>
                              </blockquote>
                              <br>
                              <br>
                              <br>
                              <hr style="border:none;color:rgb(144,144,144);min-height:1px;width:99%;background-color:rgb(176,176,176)">
                              <table style="border-collapse:collapse;border:none">
                                <tbody>
                                  <tr>
                                    <td style="border:none;padding:0px 15px 0px 8px"> <a href="http://www.avast.com/" target="_blank"> <img src="http://static.avast.com/emails/avast-mail-stamp.png" alt="Avast logo" border="0"> </a>
                                    </td>
                                    <td>
                                      <p style="color:rgb(61,77,90);font-family:Calibri,Verdana,Arial,Helvetica;font-size:12pt">
                                        Dit e-mailbericht is
                                        gecontroleerd op virussen met
                                        Avast antivirussoftware. <br>
                                        <a href="http://www.avast.com/" target="_blank">www.avast.com</a>
                                      </p>
                                    </td>
                                  </tr>
                                </tbody>
                              </table>
                              <br>
                            </div>
                          </div>
                        </div>
                        <br>
                        _______________________________________________<br>
                        Beginners mailing list<br>
                        <a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a><br>
                        <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
                        <br>
                      </blockquote>
                    </div>
                  </div>
                </div>
                <span><font color="#888888"><br>
                    <br clear="all">
                    <div><br>
                    </div>
                    -- <br>
                    <div>
                      <div dir="ltr">
                        <div>
                          <div dir="ltr">
                            <div dir="ltr">
                              <div>Regards</div>
                              <div dir="ltr">
                                <div><br>
                                </div>
                                <div>Sumit Sahrawat</div>
                              </div>
                            </div>
                          </div>
                        </div>
                      </div>
                    </div>
                  </font></span></div>
            </div>
            <br>
            _______________________________________________<br>
            Beginners mailing list<br>
            <a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a><br>
            <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
            <br>
          </blockquote>
        </div>
        <br>
      </div>
      <br>
      <fieldset></fieldset>
      <br>
      <pre>_______________________________________________
Beginners mailing list
<a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a>
</pre>
    </blockquote>
    <br>
  
<br><br>
<hr style="border:none;color:#909090;background-color:#b0b0b0;min-height:1px;width:99%">
<table style="border-collapse:collapse;border:none">
        <tbody><tr>
                <td style="border:none;padding:0px 15px 0px 8px">
                        <a href="http://www.avast.com/" target="_blank">
                                <img src="http://static.avast.com/emails/avast-mail-stamp.png" alt="Avast logo" border="0">
                        </a>
                </td>
                <td>
                        <p style="color:#3d4d5a;font-family:"Calibri","Verdana","Arial","Helvetica";font-size:12pt">
                                Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware.
                                <br><a href="http://www.avast.com/" target="_blank">www.avast.com</a>
                        </p>
                </td>
        </tr>
</tbody></table>
<br>
</div></div></div>

<br>_______________________________________________<br>
Beginners mailing list<br>
<a href="mailto:Beginners@haskell.org" target="_blank">Beginners@haskell.org</a><br>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners</a><br>
<br></blockquote></div><br></div>
</div></div></blockquote></div><br></div>