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Dear Brandon, Ozgur, et al,
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<div>Thanks very much for you explanation. This seems to be a perfectly reasonable explanation; the wrapper-types I used probably explicitly invoke typeOf with undefined. The problem here, however, is that in my actual program, I don't use ADTs, but I use GADTs,
so as to carry the context (Monad, Typeable1) with the constructor. To get to this context, I must pattern-match with the constructor. It seems hiding contexts (which I really like about GADTs) isn't "available" consistently. Oh well ;)</div>
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<div>Regards,</div>
<div>Philip</div>
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<div>On 29 Aug 2011, at 01:20, Brandon Allbery wrote:</div>
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<div dir="ltr">On Sun, Aug 28, 2011 at 18:44, Philip Holzenspies <span dir="ltr">
<<a href="mailto:pkfh@st-andrews.ac.uk">pkfh@st-andrews.ac.uk</a>></span> wrote:<br>
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instance (Typeable1 m, Monad m) => Typeable (MyADT m) where<br>
typeOf t@(MyADT _)<br>
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<div>typeOf is usually invoked with an undefined parameter; it should use types, never values. Here you've defined it to deconstruct what it's passed, which means that anything that uses it in the usual way (`typeOf (undefined :: someType)') will immediately
throw undefined.</div>
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<div>You don't need a deconstructor there; you (correctly) throw away the value, and it doesn't provide any type information not already available from the instance declaration. `typeOf t' should be good enough.</div>
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-- <br>
brandon s allbery <a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a><br>
wandering unix systems administrator (available) (412) 475-9364 vm/sms<br>
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