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Thank you Matt<br>
<p><br>
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<br>
<blockquote type="cite"
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<div class="moz-cite-prefix">-------- Original Message --------<br>
Subject: Re: [Haskell-cafe] SqlPersistT action in MonadError
issue<br>
From: Matt <a class="moz-txt-link-rfc2396E" href="mailto:parsonsmatt@gmail.com"><parsonsmatt@gmail.com></a><br>
To: Vlatko Bašić <a class="moz-txt-link-rfc2396E" href="mailto:vlatko.basic@gmail.com"><vlatko.basic@gmail.com></a><br>
Cc: haskell-cafe <a class="moz-txt-link-rfc2396E" href="mailto:Haskell-cafe@haskell.org"><Haskell-cafe@haskell.org></a><br>
Date: 16/07/18 17:25<br>
</div>
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<br>
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<div dir="ltr">Hi Vlatko,
<div><br>
</div>
<div>It depends on what you want to happen. The easiest way is
to use the runtime exception system and `try` to catch the
type of exception you want to handle:<br>
<br>
<font face="monospace, monospace">tryApp :: (MonadCatch m)
=> m a -> m (Either AppException a)<br>
tryApp = try</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="monospace, monospace">runDB :: (MonadIO m,
MonadCatch m) => SqlPersistT m a -> AppM m (Either
AppException a)<br>
runDB query = do<br>
App{..} <- ask<br>
liftIO $ tryApp $ runPool pgConf query pgPool<br>
<br>
f1 :: (MonadThrow m) => SqlPersistT m Bool<br>
f1 = throwM (ExcTest "Exception!")</font></div>
<div><font face="monospace, monospace"><br>
runF1 :: (MonadIO m, MonadCatch m) => AppM m (Either
AppException Bool)</font></div>
<div><font face="monospace, monospace">runF1 = runDB f1</font></div>
<div><font face="monospace, monospace"><br>
</font></div>
<div><font face="arial, helvetica, sans-serif">This approach
preserves the transaction rollback feature of `persistent`.
If you do not want to have that feature, then you can put
`ExceptT` on the outside, like `ExceptT AppException
(SqlPersistT m) a`, and this will give you short-circuiting
(via the outer ExceptT layer).</font></div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br clear="all">
<div>
<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">
<div>Matt Parsons</div>
</div>
</div>
</div>
<br>
<div class="gmail_quote">On Mon, Jul 16, 2018 at 2:18 AM, Vlatko
Basic <span dir="ltr"><<a
href="mailto:vlatko.basic@gmail.com" target="_blank"
moz-do-not-send="true">vlatko.basic@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
style="background-color:rgb(255,255,255);color:rgb(0,0,0)"
text="#000000" bgcolor="#FFFFFF"> Hi Matt,<br>
<br>
Thanks for taking time and giving so thorough explanation
and a suggestion. Much appreciated. :-)<br>
<br>
I figured out some of this, except for the last, main
part. I thought the problem was in not being able to
instantiate MonadError AppException IO, not that ExceptT
doesn't have MonadUnliftIO. <br>
<br>
I'm refactoring some db code and was just exploring
possible short-circuiting ways in SqlPersistT with custom
exception types, like get404. <br>
Which approach would you suggest for that?<span
class="HOEnZb"><font color="#888888"><br>
<br>
<br>
<br>
vlatko</font></span>
<div>
<div class="h5"><br>
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class="m_3754725405212285428headerSpan"
style="color:#000000">
<div class="m_3754725405212285428moz-cite-prefix">--------
Original Message --------<br>
Subject: Re: [Haskell-cafe] SqlPersistT action
in MonadError issue<br>
From: Matt <a
class="m_3754725405212285428moz-txt-link-rfc2396E"
href="mailto:parsonsmatt@gmail.com"
target="_blank" moz-do-not-send="true"><parsonsmatt@gmail.com></a><br>
To: <a
class="m_3754725405212285428moz-txt-link-abbreviated"
href="mailto:vlatko.basic@gmail.com"
target="_blank" moz-do-not-send="true">vlatko.basic@gmail.com</a><br>
Cc: haskell-cafe <a
class="m_3754725405212285428moz-txt-link-rfc2396E"
href="mailto:Haskell-cafe@haskell.org"
target="_blank" moz-do-not-send="true"><Haskell-cafe@haskell.org></a><br>
Date: 14/07/18 20:09<br>
</div>
<br>
<br>
</span>
<div dir="ltr">The type of `runPool` is given here: <a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#v:runPool"
target="_blank" moz-do-not-send="true">https://www.stackage.<wbr>org/haddock/lts-12.0/<wbr>persistent-2.8.2/Database-<wbr>Persist-Class.html#v:runPool</a>
<div><br>
</div>
<div> <a class="m_3754725405212285428gmail-def"
id="m_3754725405212285428gmail-v:runPool"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;background-color:rgb(255,255,0);color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left"
moz-do-not-send="true">runPool</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>::<span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/conduit-1.3.0.3/Conduit.html#t:MonadUnliftIO"
title="Conduit"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
target="_blank" moz-do-not-send="true">MonadUnliftIO</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>m
=> c -><span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigBackend"
title="Database.Persist.Class"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
target="_blank" moz-do-not-send="true">PersistConfigBackend</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c
m a -><span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigPool"
title="Database.Persist.Class"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
target="_blank" moz-do-not-send="true">PersistConfigPool</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c
-> m a</span></div>
<div><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
return type of `runPool`, then, is `m a`, for
some `m` satisfying `MonadUnliftIO`.</span><br>
</div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
type of `liftIO` is `liftIO :: (MonadIO m)
=> IO a -> m a`. This means that the
first argument to `liftIO` must be an `IO a`.</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">When
we say `liftIO (runPool ...)`, GHC tries to
unify the `m a` from `runPool` and the `IO a`
from `liftIO`. It is able to do this, as `IO`
is an instance of `MonadUnliftIO`. Then, the
concrete type of `runPool pgConf f1 pgPool`
becomes `IO Bool`.</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">GHC
now tries to unify the `m` in `f1 :: (MonadIO
m, MonadError AppException m) =>
SqlPersistT m Bool` with `IO`. It tries to
satisfy the constraints: `MonadIO` is easily
satisfied, but `MonadError AppException m`
triggers the problem you mention.</span></div>
<div><br>
</div>
<div>Because `ExceptT` does not have an instance
for `MonadUnliftIO`, we cannot use it with the
`m` in `SqlPersistT m Bool` directly. We can,
instead, use `mapReaderT` to push the `Either`
into the return type, like this:</div>
<div><br>
</div>
<div><font face="monospace, monospace">pushException
:: SqlPersistT (ExceptT e m) a ->
SqlPersistT m (Either e a)</font></div>
<div><font face="monospace, monospace">pushException
= mapReaderT runExceptT</font></div>
<div><br>
</div>
<div><br>
</div>
<div>Now, we can write:</div>
<div><br>
</div>
<div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><font
face="monospace, monospace">liftIO $ runPool
pgConf (pushException f1) pgPool</font></div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">This
gives us an `IO (Either AppException Bool)`,
which, after lifting, gives us `AppM IO
(Either AppException Bool)`. You can then use
`either throwError pure` to pull the
`AppException` into the `ExceptT` again.</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">---</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">I
would suggest that you reconsider this
approach, however. The `persistent` library
uses transactions and exceptions in a way that
is *almost always* what you want, but can be
surprising, and using `ExceptT` will break
this system. Transactions are automatically
rolled back on a `MonadCatch`-style exception,
but they are not automatically rolled back on
an `ExceptT`-style exception. Having a single
`AppException` type that represents errors
that can occur in database transactions *and*
the rest of your application is also going to
be a cause for unsafety and errors, as the
type cannot possibly be precise enough to
provide any safety.</div>
<br>
</div>
</div>
<div class="gmail_extra"><br clear="all">
<div>
<div
class="m_3754725405212285428gmail_signature"
data-smartmail="gmail_signature">
<div dir="ltr">
<div>Matt Parsons</div>
</div>
</div>
</div>
<br>
<div class="gmail_quote">On Sat, Jul 14, 2018 at
10:22 AM, Vlatko Basic <span dir="ltr"><<a
href="mailto:vlatko.basic@gmail.com"
target="_blank" moz-do-not-send="true">vlatko.basic@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
style="background-color:rgb(255,255,255);color:rgb(0,0,0)"
text="#000000" bgcolor="#FFFFFF">
<p>Hello,</p>
<p>I'm trying to use SqlPersistT funcs with
MonadError, but am failing in writing the
runDB correctly. <br>
</p>
<p>I tried several things, but always
getting:</p>
<p> • Couldn't match type ‘IOException’
with ‘AppException’<br>
arising from a functional
dependency between:<br>
constraint ‘MonadError
AppException IO’ arising from a use of
‘f1’<br>
instance ‘MonadError IOException
IO’ at <no location info><br>
• In the second argument of ‘runPool’,
namely ‘f1’<br>
In the second argument of ‘($)’,
namely ‘runPool pgConf f1 pgPool’<br>
In a stmt of a 'do' block: liftIO $
runPool pgConf f1 pgPool</p>
I understand there already is "instance
MonadError IOException IO" and fundep says
it can be only one for IO.
<p>How to make it compile?</p>
<br>
<p>Best regards,</p>
<p>vlatko</p>
<p> <br>
</p>
<p>Here is the minimal reproducible code:<br>
</p>
<font size="-1"><tt>module Test where</tt><tt><br>
</tt><tt><br>
</tt><tt>import Prelude</tt><tt><br>
</tt><tt>import Control.Exception.Base</tt><tt><br>
</tt><tt>import Control.Monad.Except</tt><tt><br>
</tt><tt>import Control.Monad.Trans.Reader</tt><tt><br>
</tt><tt>import
Database.Persist.Postgresql</tt><tt><br>
</tt><tt><br>
</tt><tt>data AppException =</tt><tt><br>
</tt><tt> ExcText String</tt><tt><br>
</tt><tt> | ExcIO IOException</tt><tt><br>
</tt><tt> deriving (Show)</tt><tt><br>
</tt><tt><br>
</tt><tt>type AppM m = ExceptT
AppException (ReaderT App m)</tt><tt><br>
</tt><tt>data App = App { pgConf ::
PostgresConf, pgPool :: ConnectionPool}</tt><tt><br>
</tt><tt><br>
</tt><tt>runDB :: (MonadIO m, MonadError
AppException m) => AppM m Bool<br>
</tt><tt>runDB = do</tt><tt><br>
</tt><tt> App{..} <- lift ask</tt><tt><br>
</tt><tt> liftIO $ runPool pgConf <b>f1</b>
pgPool -- error</tt><tt><br>
</tt><tt> -- liftIO $ runPool pgConf <b>f2</b>
pgPool -- OK</tt><tt><br>
</tt><tt><br>
</tt><tt><br>
</tt><tt>f1 :: (MonadIO m, MonadError
AppException m) => SqlPersistT m Bool</tt><tt><br>
</tt><tt>f1 = throwError $ ExcText
"exception"</tt><tt><br>
</tt><tt><br>
</tt><tt>f2 :: MonadIO m => SqlPersistT
m Bool</tt><tt><br>
</tt><tt>f2 = return True</tt></font><br>
<br>
</div>
<br>
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