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<p class="MsoNormal" style="margin-left:36.0pt">Oh! Is the definition of 'foo' rejected in recent versions of GHC? My 7.4.1 installation doesn't complain. -- Conal<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Yes, it is rejected.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><br>
Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> conal.elliott@gmail.com [mailto:conal.elliott@gmail.com]
<b>On Behalf Of </b>Conal Elliott<br>
<b>Sent:</b> 14 January 2013 20:52<br>
<b>To:</b> Simon Peyton-Jones<br>
<b>Cc:</b> Richard Eisenberg; glasgow-haskell-users@haskell.org; Haskell Cafe<br>
<b>Subject:</b> Re: Advice on type families and non-injectivity?<o:p></o:p></span></p>
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<p class="MsoNormal">There is a real difficulty here with type-checking 'bar'. (And that difficulty is why 'foo' is also rejected.)<o:p></o:p></p>
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<p class="MsoNormal">Oh! Is the definition of 'foo' rejected in recent versions of GHC? My 7.4.1 installation doesn't complain. -- Conal<o:p></o:p></p>
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<p class="MsoNormal">On Mon, Jan 14, 2013 at 3:39 AM, Simon Peyton-Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">| > > {-# LANGUAGE TypeFamilies #-}<br>
| > ><br>
| > > type family F a<br>
| > ><br>
| > > foo :: F a<br>
| > > foo = undefined<br>
| > ><br>
| > > bar :: F a<br>
| > > bar = foo<o:p></o:p></p>
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<p class="MsoNormal">There is a real difficulty here with type-checking 'bar'. (And that difficulty is why 'foo' is also rejected.)<br>
<br>
Namely, when typechecking 'bar', we must instantiate foo with an unknown type, say alpha. So now we must find a type 'alpha' such that<br>
F a ~ F alpha<br>
Certainly alpha=1 is one solution, but there might be others. For example, suppose<br>
type instance F [b] = F b<br>
Then alpha=[a] would also be a solution.<br>
<br>
In this particular case any solution will do, but suppose there was an addition constraint (C alpha) arising from the right hand side, where C is a class. Then if we had<br>
instance C [b] where ...<br>
then the second solution (alpha=[a]) would work, but not the first. This can get arbitrarily complicated, and GHC's type inference does not "search" various solutions; it follows one path.<br>
<br>
The solution is to provide a way to fix alpha. For example,<br>
foo :: a -> F a<br>
is fine.<br>
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<span class="hoenzb">Simon</span></span><o:p></o:p></p>
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<br>
| -----Original Message-----<br>
| From: <a href="mailto:glasgow-haskell-users-bounces@haskell.org">glasgow-haskell-users-bounces@haskell.org</a> [mailto:<a href="mailto:glasgow-haskell-">glasgow-haskell-</a><br>
| <a href="mailto:users-bounces@haskell.org">users-bounces@haskell.org</a>] On Behalf Of Richard Eisenberg<br>
| Sent: 14 January 2013 03:47<br>
| To: Conal Elliott<br>
| Cc: <a href="mailto:glasgow-haskell-users@haskell.org">glasgow-haskell-users@haskell.org</a>; Haskell Cafe<br>
| Subject: Re: Advice on type families and non-injectivity?<br>
|<br>
| Hi Conal,<br>
|<br>
| I agree that your initial example is a little puzzling, and I'm glad<br>
| that the new ambiguity checker prevents both definitions, not just one.<br>
|<br>
| However, your initial question seems broader than just this example. I<br>
| have run into this problem (wanting injective type functions) several<br>
| times myself, and have been successful at finding workarounds. But, I<br>
| can't think of any unifying principle or solid advice. If you can post<br>
| more information about your problem, perhaps I or others can contribute.<br>
|<br>
| For what it's worth, the desire for injective type functions has been<br>
| entered as ticket #6018 in the GHC Trac, but I see you're already on the<br>
| cc: list. I believe Simon PJ has given serious thought to implementing<br>
| this feature and may have even put in some very basic code toward this<br>
| end.<br>
|<br>
| Richard<br>
|<br>
| On Jan 13, 2013, at 2:10 PM, Conal Elliott <<a href="mailto:conal@conal.net">conal@conal.net</a>> wrote:<br>
|<br>
| > I sometimes run into trouble with lack of injectivity for type<br>
| families. I'm trying to understand what's at the heart of these<br>
| difficulties and whether I can avoid them. Also, whether some of the<br>
| obstacles could be overcome with simple improvements to GHC.<br>
| ><br>
| > Here's a simple example:<br>
| ><br>
| > > {-# LANGUAGE TypeFamilies #-}<br>
| > ><br>
| > > type family F a<br>
| > ><br>
| > > foo :: F a<br>
| > > foo = undefined<br>
| > ><br>
| > > bar :: F a<br>
| > > bar = foo<br>
| ><br>
| > The error message:<br>
| ><br>
| > Couldn't match type `F a' with `F a1'<br>
| > NB: `F' is a type function, and may not be injective<br>
| > In the expression: foo<br>
| > In an equation for `bar': bar = foo<br>
| ><br>
| > A terser (but perhaps subtler) example producing the same error:<br>
| ><br>
| > > baz :: F a<br>
| > > baz = baz<br>
| ><br>
| > Replacing `a` with a monotype (e.g., `Bool`) eliminates the error.<br>
| ><br>
| > Does the difficulty here have to do with trying to *infer* the type<br>
| and then compare with the given one? Or is there an issue even with type<br>
| *checking* in such cases?<br>
| ><br>
| > Other insights welcome, as well as suggested work-arounds.<br>
| ><br>
| > I know about (injective) data families but don't want to lose the<br>
| convenience of type synonym families.<br>
| ><br>
| > Thanks, -- Conal<br>
| ><br>
| > _______________________________________________<br>
| > Glasgow-haskell-users mailing list<br>
| > <a href="mailto:Glasgow-haskell-users@haskell.org">Glasgow-haskell-users@haskell.org</a><br>
| > <a href="http://www.haskell.org/mailman/listinfo/glasgow-haskell-users" target="_blank">
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users</a><br>
|<br>
|<br>
| _______________________________________________<br>
| Glasgow-haskell-users mailing list<br>
| <a href="mailto:Glasgow-haskell-users@haskell.org">Glasgow-haskell-users@haskell.org</a><br>
| <a href="http://www.haskell.org/mailman/listinfo/glasgow-haskell-users" target="_blank">
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users</a><o:p></o:p></p>
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