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Sun Oct 23 10:51:38 CEST 2011

l be equal.=C2=A0</blockquote>

<div><br></div><div>This version works as expected:</div><div class=3D"im">=
<div><br></div><div>import System.Mem.StableName</div><div>import Control.M=
onad.State</div><div><br></div><div>eq :: a -&gt; b -&gt; IO Bool</div><div=
eq a b =3D do</div>
<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0pa &lt;- makeStableNam=
e a</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0pb &lt;- make=
StableName b</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0retu=
rn (hashStableName pa =3D=3D hashStableName pb)</div><div><br></div><div>su=
ccessor :: (Num a, Monad m) =3D&gt; a -&gt; m a</div>

<div>successor n =3D return (n+1)</div><div><br></div></div><div>-- =C2=A0m=
ain :: IO ()</div><div>-- =C2=A0main =3D do</div><div>-- =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 b2 &lt;- eq (successor :: Int -&gt; State Int Int) (successor ::=
 Int -&gt; State Int Int)</div>
<div>-- =C2=A0 =C2=A0 =C2=A0 =C2=A0 b1 &lt;- eq (successor :: Int -&gt; May=
be Int) (successor :: Int -&gt; Maybe Int)</div>
<div>-- =C2=A0 =C2=A0 =C2=A0 =C2=A0 print (show b1 ++ &quot; &quot; ++ show=
 b2)</div><div class=3D"im"><div><br></div><div>main :: IO ()</div><div>mai=
n =3D do</div></div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0b2 &lt;- eq f2 f2</div>=
<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0b1 &lt;- eq f1 f1</div><div class=3D"im">
<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0print (show b1 ++ &quot; &quot; ++ show b2)=
</div><div>=C2=A0 =C2=A0where f1 =3D (successor :: Int -&gt; Maybe Int)</di=
v><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0f2 =3D (successor :: Int -&gt; Sta=
te Int Int)</div><div><br></div><div><br></div><div><br></div><div>hth,</di=
<div><br></div><br><div class=3D"gmail_quote"><div><div class=3D"h5">On Tue=
, Jun 26, 2012 at 1:15 PM, Ismael Figueroa Palet <span dir=3D"ltr">&lt;<a h=
ref=3D"mailto:ifigueroap at" target=3D"_blank">ifigueroap at<=
/a>&gt;</span> wrote:<br>
</div></div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;bo=
rder-left:1px #ccc solid;padding-left:1ex"><div><div class=3D"h5">
I&#39;m using StableNames to have a notion of function equality, and I&#39;=
m running into problems when using monadic functions.<div><br></div><div>Co=
nsider the code below, file Test.hs</div><div><br></div><div><div>import Sy=

<div>import Control.Monad.State</div><div><br></div><div>eq :: a -&gt; b -&=
gt; IO Bool</div><div>eq a b =3D do</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0pa &lt;- makeStableName a</div><div>=C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0pb &lt;- makeStableName b</div><div>=C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return (hashStableName pa =3D=3D h=
ashStableName pb)</div>

<div><br></div><div>successor :: (Num a, Monad m) =3D&gt; a -&gt; m a</div>=
<div>successor n =3D return (n+1)</div><div><br></div><div>main :: IO ()=C2=
=A0</div><div>main =3D do=C2=A0</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0b1 &lt=
;- eq (successor :: Int -&gt; Maybe Int) (successor :: Int -&gt; Maybe Int)=
 =C2=A0 =C2=A0 =C2=A0=C2=A0</div>

<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0b2 &lt;- eq (successor :: Int -&gt; State I=
nt Int) (successor :: Int -&gt; State Int Int)</div><div>=C2=A0 =C2=A0 =C2=
=A0 =C2=A0print (show b1 ++ &quot; &quot; ++ show b2)</div></div><div><br><=
/div><div>Running the code into ghci the result is &quot;False False&quot;.=
 There is some old post saying that this is due to the dictionary-passing s=
tyle for typeclasses, and compiling with optimizations improves the situati=

<div><br></div><div>Compiling with ghc --make -O Tests.hs and running the p=
rogram, the result is &quot;True True&quot;, which is what I expect.</div><=
div>However, if I change main to be like the following:</div><div><br>

<div><div>main :: IO ()=C2=A0</div><div>main =3D do=C2=A0=C2=A0 =C2=A0 =C2=
=A0 =C2=A0</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0b2 &lt;- eq (successor :: I=
nt -&gt; State Int Int) (successor :: Int -&gt; State Int Int)</div><div>=
=C2=A0 =C2=A0 =C2=A0 =C2=A0b1 &lt;- eq (successor :: Int -&gt; Maybe Int) (=
successor :: Int -&gt; Maybe Int) =C2=A0 =C2=A0 =C2=A0=C2=A0</div>

<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0print (show b1 ++ &quot; &quot; ++ show b2)=
</div></div><div><br></div><div>i.e. just changing the sequential order, an=
d then compiling again with the same command, I get &quot;True False&quot;,=
 which is very confusing for me.</div>

<div>Similar situations happens when using the state monad transformer, and=
 manually built variations of it.=C2=A0</div><div><br></div><div>It sounds =
the problem is with hidden closures created somewhere that do not point to =
the same memory locations, so StableNames yields false for that cases, but =
it is not clear to me under what circumstances this situation happens. Is t=
here other way to get some approximation of function equality? or a way to =
&quot;configure&quot; the behavior of StableNames in presence of class cons=

<div><br></div><div>I&#39;m using the latests Haskell Platform on OS X Lion=
, btw.</div><div><br></div><div>Thanks=EF=9C=A9</div><span><font color=3D"#=
888888"><div><div><div><br></div>-- <br>Ismael<br><br>
Haskell-Cafe mailing list<br>
<a href=3D"mailto:Haskell-Cafe at" target=3D"_blank">Haskell-Cafe@=</a><br>
<a href=3D"" target=3D"=
</blockquote></div><br><br clear=3D"all"><div><br></div>-- <br>Ismael<br><b=


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