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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]--> <table lang="container" border="0" cellpadding="0" cellspacing="0" valign="top" style="width:100%;margin-top:6px"><tbody><tr><td valign="top" style="line-height:1.31;font-family:arial,sans-serif"><div>Hi Everyone, </div><div><br></div><div>I am trying to set up a simple web application, but with all the trappings of a production app. (This is strictly for learning purposes). </div><div><br></div><div>At the moment, I am trying to get a database pool up and running that the rest of the application can use. You can see my current progress at <a href="https://github.com/StevenXL/todo/tree/07-add-database">this repository</a>. You may want to look at only the code in the back-end folder, as there is a whole front-end application.</div><div><br></div><div>So, again, my goal is to get a database pool up and running. At the moment, if I try to build the executable target, I get the following error:</div><div><br></div><div><div style="height:4px;font-size:4px"><br></div><table id="" cellpadding="0" cellspacing="0" style="border-width:1px;border-style:solid;border-color:rgb(245,255,255);border-radius:4px;width:100%;max-width:578px"><tbody><tr style="border:1px solid #d5ecff;mso-border-alt:none;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;position:relative"><table cellpadding="0" cellspacing="0" valign="top" width="100%" style="width:100% !important"><tbody><tr><td valign="top" align="left" width="100%" style="padding-top:6px;padding-bottom:8px;line-height:18px;font-size:13px;font-family:proxima-nova,"Avenir Next","Segoe UI",Calibri,"Helvetica Neue",Helvetica,Arial,sans-serif;min-width:100% !important"> Error </td></tr></tbody></table><div style="color:#333;font-size:16px;border-radius:3px"><div style="margin-bottom:1em;font-family:Consolas,Liberation Mono,Menlo,Courier,monospace;border:1px solid #ddd;border-radius:3px;overflow:hidden;-webkit-mask-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAA5JREFUeNpiYGBgAAgwAAAEAAGbA+oJAAAAAElFTkSuQmCC)"><div style="width:540px;overflow-x:scroll"><div style="font-size:12px;line-height:1.4"><table cellpadding="0" cellspacing="0" width="1000" style="line-height:1.4;margin-bottom:0px;background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial;width:1000px !important"><tbody><tr><td align="left" style="padding:.5em 1em"><pre style="font-family:Consolas,"Liberation Mono",Menlo,Courier,monospace;font-size:100%;line-height:inherit;background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial"> <div style="font-variant-numeric:normal;font-stretch:normal;line-height:normal;font-family:Monaco,Menlo,"Ubuntu Mono",Consolas,source-code-pro,monospace;color:rgb(77,77,76)"><div style="font-stretch:normal;line-height:normal;height:16px">/Users/stevenleiva/Repos/todo/back-end/app/Main.hs:10:13: error:</div><div style="font-stretch:normal;line-height:normal;height:16px"> • Couldn't match type ‘persistent-2.7.0:Database.Persist.Class.PersistStore.BaseBackend</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> backend0’</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> with ‘Database.Persist.Sql.Types.Internal.SqlBackend’</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> arising from a use of ‘createPostgresqlPool’</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> The type variable ‘backend0’ is ambiguous</div><div style="font-stretch:normal;line-height:normal;height:16px"> • In a stmt of a 'do' block: pool <- createPostgresqlPool connStr 1</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> In the expression:</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> do { pool <- createPostgresqlPool connStr 1;</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> startApp }</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> In an equation for ‘main’:</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> main</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> = do { pool <- createPostgresqlPool connStr 1;</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> startApp }</div></div>
</pre></td></tr></tbody></table></div></div></div></div><table width="100%" cellpadding="0" cellspacing="0" valign="top" style="border-top:1px solid rgb(221,221,238);table-layout:fixed;min-width:100% !important"><tbody><tr><td valign="top" style="padding-top:8px"><a href="https://mixmax.com/r/59ec918e83319a2a077ff18c" style="display:inline-block;vertical-align:top" target="_blank"><img src="https://emailapps.mixmax.com/img/badge_mixmax.png" align="top" height="20" style="display:block" alt="Mixmax" border="0"></a> <a style="color:rgb(170,170,187);vertical-align:top;padding-top:4px;display:inline-block;font-size:11px;line-height:16px;margin:0px;letter-spacing:1px;padding-left:6px;text-transform:uppercase;font-family:"Avenir Next","Segoe UI",Calibri,Arial,sans-serif" href="https://mixmax.com/r/59ec918e83319a2a077ff18c" target="_blank">Not using Mixmax yet?</a> </td></tr></tbody></table></td></tr></tbody></table><div style="height:4px;font-size:4px"><br></div></div><div><br></div><div>I tried helping out the compiler by adding <b>pool :: Pool Connection</b>, after importing those type constructors / constants. At that point, I got a different error:</div><div><br></div><div><div style="height:4px;font-size:4px"><br></div><table id="" cellpadding="0" cellspacing="0" style="border-width:1px;border-style:solid;border-color:rgb(245,255,255);border-radius:4px;width:100%;max-width:578px"><tbody><tr style="border:1px solid #d5ecff;mso-border-alt:none;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;position:relative"><table cellpadding="0" cellspacing="0" valign="top" width="100%" style="width:100% !important"><tbody><tr><td valign="top" align="left" width="100%" style="padding-top:6px;padding-bottom:8px;line-height:18px;font-size:13px;font-family:proxima-nova,"Avenir Next","Segoe UI",Calibri,"Helvetica Neue",Helvetica,Arial,sans-serif;min-width:100% !important"> Error 2 </td></tr></tbody></table><div style="color:#333;font-size:16px;border-radius:3px"><div style="margin-bottom:1em;font-family:Consolas,Liberation Mono,Menlo,Courier,monospace;border:1px solid #ddd;border-radius:3px;overflow:hidden;-webkit-mask-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAA5JREFUeNpiYGBgAAgwAAAEAAGbA+oJAAAAAElFTkSuQmCC)"><div style="width:540px;overflow-x:scroll"><div style="font-size:12px;line-height:1.4"><table cellpadding="0" cellspacing="0" width="1000" style="line-height:1.4;margin-bottom:0px;background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial;width:1000px !important"><tbody><tr><td align="left" style="padding:.5em 1em"><pre style="font-family:Consolas,"Liberation Mono",Menlo,Courier,monospace;font-size:100%;line-height:inherit;background-image:initial;background-position:initial;background-size:initial;background-repeat:initial;background-attachment:initial;background-origin:initial;background-clip:initial"> <div style="font-variant-numeric:normal;font-stretch:normal;line-height:normal;font-family:Monaco,Menlo,"Ubuntu Mono",Consolas,source-code-pro,monospace;color:rgb(77,77,76)"><div style="font-stretch:normal;line-height:normal;height:16px">/Users/stevenleiva/Repos/todo/back-end/app/Main.hs:11:32: error:</div><div style="font-stretch:normal;line-height:normal;height:16px"> • No instance for (monad-logger-0.3.25.1:Control.Monad.Logger.MonadLogger</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> IO)</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> arising from a use of ‘createPostgresqlPool’</div><div style="font-stretch:normal;line-height:normal;height:16px"> • In a stmt of a 'do' block:</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> pool :: Pool Connection <- createPostgresqlPool connStr 1</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> In the expression:</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> do { pool :: Pool Connection <- createPostgresqlPool connStr 1;</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> startApp }</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> In an equation for ‘main’:</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> main</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> = do { pool :: Pool Connection <- createPostgresqlPool connStr 1;</div><div style="font-stretch:normal;line-height:normal;height:16px"><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span><span style="font-variant-numeric:normal;font-stretch:normal;line-height:normal"> </span> startApp }</div></div>
</pre></td></tr></tbody></table></div></div></div></div><table width="100%" cellpadding="0" cellspacing="0" valign="top" style="border-top:1px solid rgb(221,221,238);table-layout:fixed;min-width:100% !important"><tbody><tr><td valign="top" style="padding-top:8px"><a href="https://mixmax.com/r/59ec918e83319a2a077ff18c" style="display:inline-block;vertical-align:top" target="_blank"><img src="https://emailapps.mixmax.com/img/badge_mixmax.png" align="top" height="20" style="display:block" alt="Mixmax" border="0"></a> <a style="color:rgb(170,170,187);vertical-align:top;padding-top:4px;display:inline-block;font-size:11px;line-height:16px;margin:0px;letter-spacing:1px;padding-left:6px;text-transform:uppercase;font-family:"Avenir Next","Segoe UI",Calibri,Arial,sans-serif" href="https://mixmax.com/r/59ec918e83319a2a077ff18c" target="_blank">Not using Mixmax yet?</a> </td></tr></tbody></table></td></tr></tbody></table><div style="height:4px;font-size:4px"><br></div></div><div><font face="Arial, sans-serif" color="#222222"><br></font></div><div><font face="Arial, sans-serif" color="#222222">I understand that <b>createPostgresqlPool</b> eventually gives me a value of type <b>m (Pool Connection)</b>, where there are some constraints on <b>m. </b>I also understand that at the moment I am getting back a value <b>IO (Pool Connection)</b>, and that <b>IO</b> does not have an instance of <b>MonadLogger</b>. </font></div><div><font face="Arial, sans-serif" color="#222222"><br></font></div><div><font face="Arial, sans-serif" color="#222222">From here, I am at a bit of a lose on how to proceed. I thought implementing an instance of MonadLogger for IO that would simply typecheck (and not log), but that would lead to an <b>orphaned instance</b>, correct? What's the easiest next step to make forward progress here? </font></div><div><font face="Arial, sans-serif" color="#222222"><br></font></div><div><font face="Arial, sans-serif" color="#222222">P.S. I am aware that there are a lot of advanced (for me anyway) Haskell that can be done via monad transformers, natural transformations, etc., in order to wire up a DB with Servant, but I am happy with the "next simplest step" until I learn some of those concepts better. (Then Haskell makes refactoring a breeze!).</font></div><div><br></div><div>Steven Leiva<br><span id="gc-number-0" title="Call with Google Voice"><span id="gc-number-0" class="gc-cs-link" title="Call with Google Voice">305.528.6038</span></span><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="width:0px;height:0px" src="https://track.mixmax.com/api/track/v2/lnvyrO08QxJqHcV6q/i02bj5CbpFWbnBkblZXZ0NnLhZXalxmI/i02bj5CbpFWbnBkblZXZ0NnLhZXalxmI/ISY2lWZMBiblZXZ0NlI?sc=true" alt=""></td></tr></tbody></table><div> </div><div><br></div><div class="mixmax_signature">Steven Leiva<br><span><span id="gc-number-1" class="gc-cs-link" title="Call with Google Voice">305.528.6038</span></span><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/pGsp2pniftrtWGIsg/i02bj5CbpFWbnBkblZXZ0NnLhZXalxmI/IyZy9mLsxWZrNXYoBUZmF2YtwGblt2chhmI/ISZmF2YtwGblt2chhmI?sc=false" alt=""> <!--[if mso]></td></tr></table><![endif]--> </td> </tr> </table> </body> </html>