[Haskell-cafe] Unexpected Typeable behaviour; Prelude.undefined
allbery.b at gmail.com
Mon Aug 29 01:20:43 CEST 2011
On Sun, Aug 28, 2011 at 18:44, Philip Holzenspies <pkfh at st-andrews.ac.uk>wrote:
> instance (Typeable1 m, Monad m) => Typeable (MyADT m) where
> typeOf t@(MyADT _)
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.
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.
brandon s allbery allbery.b at gmail.com
wandering unix systems administrator (available) (412) 475-9364 vm/sms
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