<div dir="ltr"><div></div>I've tested with ghc-7.8.4, and the test1/test2 definitions are accepted if you use -XNoMonomorphismRestriction.<br><br>For test2 to work without annotating a result type, you could use a superclass constraint to show that the type family has an inverse. In ghc-7.10 it doesn't work (see <a href="https://ghc.haskell.org/trac/ghc/ticket/10009">https://ghc.haskell.org/trac/ghc/ticket/10009</a>)<br><br><a href="https://gist.github.com/aavogt/7a10024f0199dc2e8478">https://gist.github.com/aavogt/7a10024f0199dc2e8478</a> shows both features.<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Sep 10, 2015 at 10:17 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">One possible fix (tested on GHC-7.10.1 with lens-4.12.3):<div><br></div><div><div><font face="monospace, monospace">test2 :: (Test a, t ~ TestT a) => t -> a</font></div><span class=""><div><font face="monospace, monospace">test2 = view (from myiso)</font></div></span></div><div><br></div><div>This might have something to do with type families not being injective, but I'm not completely sure.</div><div><br></div><div>I also agree that it might be possible to trigger this without lens, will try to find an example and post if I succeed.</div></div><div class="gmail_extra"><div><div class="h5"><br><div class="gmail_quote">On 11 September 2015 at 05:28, Nikolay Amiantov <span dir="ltr"><<a href="mailto:ab@fmap.me" target="_blank">ab@fmap.me</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Cafe,<br>
<br>
I've been playing around with lens and stumbled upon strange GHC<br>
behaviour. Consider this source (using lens package and GHC 7.10.2):<br>
<br>
{-# LANGUAGE TypeFamilies #-}<br>
<br>
import Control.Lens<br>
<br>
class Test a where<br>
type TestT a<br>
myiso :: Iso' a (TestT a)<br>
<br>
test1 :: Test a => a -> TestT a<br>
test1 = view myiso<br>
<br>
test2 :: Test a => TestT a -> a<br>
test2 = view (from myiso)<br>
<br>
GHC would emit this error:<br>
<br>
/tmp/test.hs:13:9:<br>
Could not deduce (Control.Monad.Reader.Class.MonadReader<br>
(TestT a) ((->) (TestT a)))<br>
arising from a use of ‘view’<br>
from the context (Test a)<br>
bound by the type signature for test2 :: Test a => TestT a -> a<br>
at /tmp/test.hs:12:10-31<br>
In the expression: view (from myiso)<br>
In an equation for ‘test2’: test2 = view (from myiso)<br>
Failed, modules loaded: none.<br>
<br>
However, `MonadReader r ((->) r)` is defined for any and all `r`!<br>
Furthermore, `test1` has no problem with this and `view` there uses this<br>
instance too. The only difference that I see is the presence of a type<br>
family:<br>
<br>
* `test1` needs `MonadReader a ((->) a)`<br>
* `test2` needs `MonadReader (TestT a) ((->) (TestT a))`<br>
<br>
, but I don't understand how can this result in a different behavior.<br>
Notice that this likely may be reproduced somehow without lens -- I've<br>
spent some time trying to minify this example further but alas to no avail.<br>
<br>
Thanks in advance!<br>
<span><font color="#888888"><br>
--<br>
Nikolay.<br>
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</font></span></blockquote></div><br><br clear="all"><div><br></div></div></div><span class="HOEnZb"><font color="#888888">-- <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>
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