Re: [Haskell-cafe] ambiguous types although 'asTypeOf'

2007-12-27 Thread Henning Thielemann 

On Tue, 25 Dec 2007, Felipe Lessa wrote:

 On Dec 25, 2007 4:27 PM, Henning Thielemann
 [EMAIL PROTECTED] wrote:
  test :: (Integral a, RealFrac a) = a
  test =
 let c = undefined
 in  asTypeOf (round c) c
 
 
  When compiling I get:
 
  Compiling StorableInstance ( src/StorableInstance.hs, interpreted )
 
  src/StorableInstance.hs:38:17:
  Warning: Defaulting the following constraint(s) to type `Double'
   `RealFrac a' arising from use of `round' at 
  src/StorableInstance.hs:38:17-21
   In the first argument of `asTypeOf', namely `(round c)'
   In the definition of `test1': test1 = let c = undefined in 
  asTypeOf (round c) c

 Interesting, I don't see this behaviour at all.

Of course, I use -Wall all the time. :-)
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[Haskell-cafe] ambiguous types although 'asTypeOf'

2007-12-25 Thread Henning Thielemann 

I thought I understand monomorphism restriction. But it seems, I don't. I
have boilt down my problem to the following test. Don't try to make any
sense of it, it is just the smallest code I could come up with.


test :: (Integral a, RealFrac a) = a
test =
   let c = undefined
   in  asTypeOf (round c) c


When compiling I get:

Compiling StorableInstance ( src/StorableInstance.hs, interpreted )

src/StorableInstance.hs:38:17:
Warning: Defaulting the following constraint(s) to type `Double'
 `RealFrac a' arising from use of `round' at 
src/StorableInstance.hs:38:17-21
 In the first argument of `asTypeOf', namely `(round c)'
 In the definition of `test1': test1 = let c = undefined in 
asTypeOf (round c) c
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Re: [Haskell-cafe] ambiguous types although 'asTypeOf'

2007-12-25 Thread Andrea Vezzosi 
As i understand it, the monomorphism restriction applies to constrained
type variables, but c :: forall a. a, so it remains polymorphic and each of
its uses can be instantiated to a different type.

2007/12/25, Henning Thielemann [EMAIL PROTECTED]:


 I thought I understand monomorphism restriction. But it seems, I don't. I
 have boilt down my problem to the following test. Don't try to make any
 sense of it, it is just the smallest code I could come up with.


 test :: (Integral a, RealFrac a) = a
 test =
let c = undefined
in  asTypeOf (round c) c


 When compiling I get:

 Compiling StorableInstance ( src/StorableInstance.hs, interpreted )

 src/StorableInstance.hs:38:17:
 Warning: Defaulting the following constraint(s) to type `Double'
  `RealFrac a' arising from use of `round' at
 src/StorableInstance.hs:38:17-21
  In the first argument of `asTypeOf', namely `(round c)'
  In the definition of `test1': test1 = let c = undefined in
 asTypeOf (round c) c
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 Haskell-Cafe@haskell.org
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Re: [Haskell-cafe] ambiguous types although 'asTypeOf'

2007-12-25 Thread Felipe Lessa 
On Dec 25, 2007 4:27 PM, Henning Thielemann
[EMAIL PROTECTED] wrote:
 test :: (Integral a, RealFrac a) = a
 test =
let c = undefined
in  asTypeOf (round c) c


 When compiling I get:

 Compiling StorableInstance ( src/StorableInstance.hs, interpreted )

 src/StorableInstance.hs:38:17:
 Warning: Defaulting the following constraint(s) to type `Double'
  `RealFrac a' arising from use of `round' at 
 src/StorableInstance.hs:38:17-21
  In the first argument of `asTypeOf', namely `(round c)'
  In the definition of `test1': test1 = let c = undefined in 
 asTypeOf (round c) c

Interesting, I don't see this behaviour at all.

$ cat t.hs
module Main where

instance Integral Double

test :: (Integral a, RealFrac a) = a
test =
  let c = undefined
  in  asTypeOf (round c) c

main = print (test :: Double)

$ ghc --make t.hs
[1 of 1] Compiling Main ( t.hs, t.o )

t.hs:3:0:
Warning: No explicit method nor default method for `quotRem'
 In the instance declaration for `Integral Double'

t.hs:3:0:
Warning: No explicit method nor default method for `toInteger'
 In the instance declaration for `Integral Double'
Linking t ...

$ ./t
t: Prelude.undefined

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.6.1


-- 
Felipe.
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