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<div>So this works, what i find surprising is that type families do this at all...in my head "type" means some sort of synonym, so to allow unmapped synonyms and not flag them as an error seems very odd.</div>
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<div>With datakind its doubly troubling as the unmatched family expression is of the same kind as the intended types in the domain of the "function"...so its unmatched and ive extended what seemed to be a closed expression defining some types of kind N.</div>
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Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage.
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On 21 Jun 2015, at 13:07, Nicholls, Mark <<a href="mailto:nicholls.mark@vimn.com">nicholls.mark@vimn.com</a>> wrote:<br>
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<p>ugly...but effective.</p>
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<p>thanks</p>
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<div id="divRpF33800" style="DIRECTION: ltr"><font color="#000000" size="2" face="Tahoma"><b>From:</b> adam vogt [<a href="mailto:vogt.adam@gmail.com">vogt.adam@gmail.com</a>]<br>
<b>Sent:</b> 20 June 2015 16:34<br>
<b>To:</b> Richard Eisenberg<br>
<b>Cc:</b> haskell-cafe; Nicholls, Mark<br>
<b>Subject:</b> Re: [Haskell-cafe] Mild confusion around type family<br>
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On Jun 19, 2015 2:29 PM, "Richard Eisenberg" <<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>> wrote:<br>
> With a closed family, you could always do something like this:<br>
><br>
> > type family Error (msg :: Symbol) -- no instances!<br>
> > type family Sub a b where<br>
> > -- real work<br>
> > Sub a b = Error "Sub is undefined here"<br>
><br>
> Would that help? It still doesn't immediately error (there's no way to do that currently), but it's perhaps more telling to have an error string.</p>
<p dir="ltr">Ghc does error out right away if you give it an infinite loop:</p>
<p dir="ltr">type family Fail (x::k) (a :: *) where<br>
Fail x a = Fail x (Fail "don't raise the context stack please" a)</p>
<p dir="ltr">-- defining f leads to compile failure, and the message includes the "msg"<br>
f :: Fail "msg" ()<br>
f = undefined</p>
<p dir="ltr">But it would be nice to have a less hacky solution, especially as this one gives error messages that suggest giving the type checker more resources.</p>
<p dir="ltr">Regards,<br>
Adam<br>
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