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<![endif]--> <table class="container" lang="container" dir="ltr" border="0" cellpadding="0" cellspacing="0" valign="top" style="width:100%;margin-top:6px"> <tr> <td valign="top" class="message-wrapper" style="line-height:1.31;color:#222;font-family:arial,sans-serif"> <!--[if mso]><table border="0" cellpadding="0" cellspacing="0" valign="top" style="border-collapse:separate;"><tr><td valign="top"><![endif]--> <div><br></div><table border="0" cellpadding="0" cellspacing="0" style="color:rgb(0,0,0);font-family:arial,sans-serif;font-size:12.8px;width:1022px;margin-top:6px"><tbody><tr><td style="line-height:1.31;color:rgb(34,34,34)"><p>Hello everyone,</p><p><br></p><p>I am watching a talk on YouTube called <a href="https://www.youtube.com/watch?v=8t8fjkISjus">Wrangling Monad Transformer Stacks</a>.</p><p><br></p><p>The talk has the following code:</p><p><br></p><p><br></p><table cellpadding="0" cellspacing="0" style="border-width:1px;border-style:solid;border-color:rgb(245,255,255);border-radius:4px;width:576px;max-width:578px"><tbody><tr style="border:1px solid rgb(213,236,255);display:block;border-radius:3px"><td style="display:block;padding:8px;border-radius:2px;border-width:1px;border-style:solid;border-color:rgb(153,176,225);font-size:0px;vertical-align:top"><table cellpadding="0" cellspacing="0" width="100%" style="width:556px"><tbody><tr><td width="100%" style="font-family:proxima-nova,"Avenir Next","Segoe UI",Calibri,"Helvetica Neue",Helvetica,Arial,sans-serif;padding-top:6px;padding-bottom:8px;line-height:18px;font-size:13px;min-width:100%">code</td></tr></tbody></table><table cellpadding="0" cellspacing="0" width="1000" style="font-family:Consolas,"Liberation Mono",Menlo,Courier,monospace;font-size:12px;line-height:1.4;margin-bottom:0px;color:rgb(51,51,51);background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-origin:initial;background-clip:initial;width:1000px !important"><tbody><tr><td style="font-family:arial,sans-serif;padding:0.5em 1em"><p>{-# LANGUAGE GeneralizedNewtypeDeriving -#}</p><p></p><p>module DBTrans where</p><p></p><p>newtype WithTrans a = WithTrans (WithDBConn a) deriving (Functor, Applicative, Monad)</p><p></p><p>inDBTrans :: WithTrans a -> WithConn a</p><p>inDBTrans = ...</p><p></p></td></tr></tbody></table><table width="100%" cellpadding="0" cellspacing="0" style="border-top:1px solid rgb(221,221,238);table-layout:fixed;min-width:100%"><tbody><tr><td style="padding-top:8px"><a href="https://mixmax.com/r/59ec918e83319a2a077ff18c"><img src="https://emailapps.mixmax.com/img/badge_mixmax.png" height="20"></a> <a href="https://mixmax.com/r/59ec918e83319a2a077ff18c">NOT USING MIXMAX YET?</a></td></tr></tbody></table></td></tr></tbody></table><p><br></p><p>The speaker mentions that by using a newtype, we can perform "access control". The function <b>inDBTrans</b> is "aware" that the type constructor <b>WithTrans</b> is simply a wrapper for <b>WithDBConn</b>, and that functions outside of the module are not privy to that information. </p><p><br></p><p></p><p><font color="#888888"></font></p><p></p><p>Out of necessity, the speaker has to assume a certain level of Haskell knowledge that I don't have, because I am confused as to what language principles would allow for <b>inDBTrans</b> to be used for access control.</p><p><br></p><p>I understand that a newtype declaration does create a new type from a "type safey" point of view; that a newtype declaration is required to have a single data constructor with a single (not named) field; and that, once the type-checking is done, the compiler can strip out the newtype wrapper and leave the underlying type (informally speaking, of course), but that still doesn't get me to <b>inDBTrans</b> can do access control.</p><p><br></p><p>Thank you for any help!</p></td></tr></tbody></table><div class="mixmax_signature">Steven Leiva<br>305.528.6038<br><a href="mailto:leiva.steven@gmail.com" target="_blank">leiva.steven@gmail.com</a><br><a href="http://www.linkedin.com/in/stevenleiva" target="_blank">http://www.linkedin.com/in/stevenleiva</a><br></div><img align="left" width="0" height="0" style="border:0;width:0px;height:0px" src="https://track.mixmax.com/api/track/v2/THQy50tx4l1QopGIV/i02bj5CbpFWbnBkblZXZ0NnLhZXalxmI/IyZy9mLsxWZrNXYoBUZmF2YtwGblt2chhmI/ISZmF2YtwGblt2chhmI?sc=false" alt=""> <!--[if mso]></td></tr></table><![endif]--> </td> </tr> </table> </body> </html>