RE: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Simon Peyton-Jones]

 
| instance Typeable a = Typeable (MVar a) where
|   typeOf x =
| mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf
(undefined::a)]

As I think you now know, the above declaration is fine if you use
-fglasgow-exts.  Haskell 98 does not support lexically scoped type
variables, but GHC -fglasgow-exts does.  You can read about it in the
GHC user manual.  

Type variables are bound by instance declarations, as above, but you can
also bind them in patterns.  So as others have said an alternative is

| instance Typeable a = Typeable (MVar a) where
|   typeOf (x::MVar b) =
| mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf
(undefined::b)]

But that's not legal in Haskell 98 either, and since 'a' is already in
scope with -fglasgow-exts, the extra binding for 'b' seems less nice.

The best Haskell 98 solution is the one Remi gives:

instance Typeable a = Typeable (MVar a) where
typeOf v= mkAppTy (mkTyCon Control.Concurrent.MVar.MVar)
[typeOf (t v)]
where
t   :: a b - b
t   = undefined

Simon

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[Haskell-cafe] Making MVar and Chan Instances of Typeable

[Benjamin Franksen]

Hello Experts,

I need MVar and Chan to be instances of Typeable. Any hint on how this is most 
easily done would be greatly appreciated. I could change the libraries and 
add 'deriving Typeable' but I hesitate to do so.

Cheers,
Ben
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Tomasz Zielonka]

On Fri, Nov 05, 2004 at 01:57:53PM +0100, Benjamin Franksen wrote:
 Hello Experts,
 
 I need MVar and Chan to be instances of Typeable. Any hint on how this is most 
 easily done would be greatly appreciated. I could change the libraries and 
 add 'deriving Typeable' but I hesitate to do so.

The easiest way is to hide type constructor Chan:

  import Control.Concurrent
  import Data.Generics

  newtype MyChan a = MyChan (Chan a) deriving Typeable

Of course, you can also write the instance for Chan by hand.

Best regards,
Tom

-- 
.signature: Too many levels of symbolic links
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Benjamin Franksen]

On Friday 05 November 2004 15:51, you wrote:
 On Fri, Nov 05, 2004 at 01:57:53PM +0100, Benjamin Franksen wrote:
  Hello Experts,
 
  I need MVar and Chan to be instances of Typeable. Any hint on how this is
  most easily done would be greatly appreciated. I could change the
  libraries and add 'deriving Typeable' but I hesitate to do so.

 The easiest way is to hide type constructor Chan:

   import Control.Concurrent
   import Data.Generics

   newtype MyChan a = MyChan (Chan a) deriving Typeable

 Of course, you can also write the instance for Chan by hand.

This might be the easiest way, but is otherwise inconvenient. I tried to write 
the instances by hand. My first attempt was:

instance Typeable a = Typeable (MVar a) where
  typeOf x =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::a)]

but unfortunately this doesn't work. Ghc complains about 

Ambiguous type variable `a1' in the top-level constraint:
  `Typeable a1' arising from use of `typeOf' at Helpers.hs:8

The reason is apparently that inside the definition of typeOf the type 
variable 'a' is not unified with the 'a' from the instance header. I could 
write 

  typeOf (MVar x) =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf y]
where
  y = undefined `asTypeOf` x

but the doc says that typeOf should be written without evaluating its 
argument, so that is ca be passed 'undefined'.

What I need is a trick that enables me to get at the type of the 'a' in the 
instance header for use inside definition of 'typeOf'.

Ben
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[MR K P SCHUPKE]

nstance Typeable a = Typeable (MVar a) where
  typeOf (x::x) =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::x)]

Keean.
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Benjamin Franksen]

On Friday 05 November 2004 16:20, MR K P SCHUPKE wrote:
 nstance Typeable a = Typeable (MVar a) where
   typeOf (x::x) =
 mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf
 (undefined::x)]

I may be missing something but this look like an open recursion to me. The 
type 'x' is 'MVar a', but what is needed is the 'a'.

Ben
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Benjamin Franksen]

On Friday 05 November 2004 13:57, Benjamin Franksen wrote:
 Hello Experts,

 I need MVar and Chan to be instances of Typeable. Any hint on how this is
 most easily done would be greatly appreciated. I could change the libraries
 and add 'deriving Typeable' but I hesitate to do so.

Ok, I found a solution but it is horrible!

module Helpers where

import Control.Concurrent
import Data.Typeable
import Foreign

instance Typeable a = Typeable (MVar a) where
  typeOf x =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf y]
where
  y = unsafePerformIO $ do
z - newEmptyMVar = readMVar
return (z `asTypeOf` x)

I dearly hope this can be done in a less convoluted fashion.

Ben
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Koji Nakahara]

On Fri, 5 Nov 2004 14:43:55 +0100
Benjamin Franksen [EMAIL PROTECTED] wrote:
snip
 the instances by hand. My first attempt was:
 
 instance Typeable a = Typeable (MVar a) where
   typeOf x =
 mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::a)]
 
 but unfortunately this doesn't work. Ghc complains about 
 
 Ambiguous type variable `a1' in the top-level constraint:
   `Typeable a1' arising from use of `typeOf' at Helpers.hs:8
 
 The reason is apparently that inside the definition of typeOf the type 
 variable 'a' is not unified with the 'a' from the instance header. I could 
 write 


You can write:

instance Typeable a = Typeable (MVar a) where
typeOf (x :: MVar a) =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::a)]


Hope it helps,
Koji Nakahara
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[MR K P SCHUPKE]

My mistake:

instance Typeable a = Typeable (MVar a) where
  typeOf (x::MVar x) =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::x)]

Keean.
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Benjamin Franksen]

On Friday 05 November 2004 15:07, Benjamin Franksen wrote:
 instance Typeable a = Typeable (MVar a) where
   typeOf x =
 mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf y]
 where
   y = unsafePerformIO $ do
 z - newEmptyMVar = readMVar
 return (z `asTypeOf` x)

which is wrong because it also passes the typeOf of the MVar and not the 
content. This one is correct, I hope:

instance Typeable a = Typeable (MVar a) where
  typeOf x =
mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf v]
where
  v = unsafePerformIO $ do
y - newEmptyMVar
readMVar (y `asTypeOf` x)

On Friday 05 November 2004 16:44, Koji Nakahara wrote:
 instance Typeable a = Typeable (MVar a) where
 typeOf (x :: MVar a) =
   mkAppTy (mkTyCon Control.Concurrent.MVar.MVar) [typeOf (undefined::a)]

Yes, that's it. The above is a lot more convoluted but has a small advantage: 
it doesn't need -fglasgow-exts.

I understand now, why pattern signatures were deemed a useful feature!

Thanks to all who helped.

Cheers,
Ben
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Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

[Remi Turk]

On Fri, Nov 05, 2004 at 01:57:53PM +0100, Benjamin Franksen wrote:
 Hello Experts,
 
 I need MVar and Chan to be instances of Typeable. Any hint on how this is most 
 easily done would be greatly appreciated. I could change the libraries and 
 add 'deriving Typeable' but I hesitate to do so.
 
 Cheers,
 Ben

It can be done in Haskell 98 the same way `asTypeOf' is defined
in the Report:

instance Typeable a = Typeable (MVar a) where
typeOf v= mkAppTy (mkTyCon Control.Concurrent.MVar.MVar)
[typeOf (t v)]
where
t   :: a b - b
t   = undefined

Groetjes,
Remi

-- 
Nobody can be exactly like me. Even I have trouble doing it.
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