Re: [Haskell-cafe] Unexpected Typeable behaviour; Prelude.undefined

[Philip Holzenspies]

Dear Brandon, Ozgur, et al,

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 ;)

Regards,
Philip


On 29 Aug 2011, at 01:20, Brandon Allbery wrote:

On Sun, Aug 28, 2011 at 18:44, Philip Holzenspies 
mailto:p...@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  
allber...@gmail.com<mailto:allber...@gmail.com>
wandering unix systems administrator (available) (412) 475-9364 vm/sms


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[Haskell-cafe] Unexpected Typeable behaviour; Prelude.undefined

[Philip Holzenspies]

L.S.,

I'm currently writing code where I want some ADTs to be parametric with a 
monad. There are some extra conditions that I place on my parameter, but I've 
boiled things down to minimal reproducibility. When I define MyADT as follows:


import Data.Typeable

data MyADT m = MyADT (m ())

instance (Typeable1 m, Monad m) => Typeable (MyADT m) where
typeOf t@(MyADT _)
 = mkTyCon "MyADT"
  `mkTyConApp`
  [typeOf1 ((return :: Monad m => MyADT m -> m (MyADT m)) t)]


it compiles fine and GHCi works as expected on a single instance of MyADT:


*Main> typeOf (MyADT (return () :: IO ()))
MyADT IO


However, as soon as I place my data type in a structure of sorts, things break 
down:


*Main> typeOf [(MyADT (return () :: IO ()))]
*** Exception: Prelude.undefined
*Main> typeOf (Just (MyADT (return () :: IO (
*** Exception: Prelude.undefined
*Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO (
*** Exception: Prelude.undefined


But if I stick a number in a similar structure, typeOf works just fine:


*Main> typeOf ((return :: a -> IO a) 5)
IO Integer


I don't quite understand where  the undefined comes from. I'm certainly not 
using it anywhere (as shown by the minimal reproduction above). Weirder still, 
when I *do* introduce a nice error, that too does not come up, viz.


import Data.Typeable

data MyADT m = MyADT (m ())
instance (Typeable1 m, Monad m) => Typeable (MyADT m) where
typeOf t@(MyADT _)
 = error "foobar"


with GHCi-session:


*Main> typeOf (MyADT (return () :: IO ()))
*** Exception: foobar
*Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO (
*** Exception: Prelude.undefined
*Main> typeOf $ Just (MyADT (return () :: IO ()))
*** Exception: Prelude.undefined
*Main> typeOf $ Just 42
Maybe Integer


FYI, I'm using GHC 7.0.3, as installed with the Haskell Platform 2011.2.0.1. Am 
I overlooking something? Any help would be appreciated.

Regards,
Philip
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