Re: [Haskell-cafe] sizeOf on a type
On Fri, 25 Dec 2009, Lennart Augustsson wrote: sizeOfPtr :: Ptr a - Int sizeOfPtr = sizeOf . (undefined :: Ptr a - a) No need for scoped type variables. But it does assume sizeOf does not use its argument. That's even better. I'll add that to the Wiki. http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Type_Variables___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
On Fri, 18 Dec 2009, Gregory Crosswhite wrote: Yay, advancePtr is exactly what I needed! I totally missed that one in the docs. Also thanks to those of you who pointed me to the scoped type variables feature, since I had figured that a feature liked that had to exist but I just didn't know where to look for it. You do not need the Scoped Type Variables feature here, although it may simplify writing. I remember we had a similar thread here, recently. A solution can be: sizeOfPtr :: Ptr a - a - Int sizeOfPtr _ x = sizeOf x to be called by (sizeOfPtr ptr undefined). I have added this example to: http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Type_Variables But in order to be found in Category:FAQ we might need a different title. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
Sean Leather wrote: I can see what's going on: it can't tell that the a I am writing there is the same a that's in the type specification, but is there any way that I can make it identify a with the a in the specification for nextPtr? Lexically scoped type variables: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#scoped-type-variables And if you require H98+FFI only, you can also use the following: ptrSizeOf = sizeOf . asTypeOf undefined . unsafePerformIO . peek The asTypeOf function is the H98 way of handling many scoped type variable issues. It never evaluates its second argument so the unsafePerformIO is perfectly safe since the peek will never be evaluated either (and, heck, the whole argument to sizeOf won't be evaluated either). -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] sizeOf on a type
Hey everyone, I would like to write a routine like nextPtr :: Storable a = Ptr a - Ptr a nextPtr = (`plusPtr` sizeOf (undefined :: a)) which increments a pointer by the correct amount, but GHC complains that the type variable a is ambiguous. I can see what's going on: it can't tell that the a I am writing there is the same a that's in the type specification, but is there any way that I can make it identify a with the a in the specification for nextPtr? Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
I would like to write a routine like nextPtr :: Storable a = Ptr a - Ptr a nextPtr = (`plusPtr` sizeOf (undefined :: a)) How about: getA :: Ptr a - a getA _ = undefined nextPtr ptr = (`plusPtr` sizeOf (getA ptr)) ptr -Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
I can see what's going on: it can't tell that the a I am writing there is the same a that's in the type specification, but is there any way that I can make it identify a with the a in the specification for nextPtr? Lexically scoped type variables: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#scoped-type-variables Regards, Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
Hi, I would like to write a routine like nextPtr :: Storable a = Ptr a - Ptr a nextPtr = (`plusPtr` sizeOf (undefined :: a)) which increments a pointer by the correct amount, but GHC complains that the type variable a is ambiguous. Maybe Foreign.Marshal.Array.advancePtr is what you really need in this case. Gergely -- http://www.fastmail.fm - The professional email service ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sizeOf on a type
Yay, advancePtr is exactly what I needed! I totally missed that one in the docs. Also thanks to those of you who pointed me to the scoped type variables feature, since I had figured that a feature liked that had to exist but I just didn't know where to look for it. You guys rock! Cheers, Greg On Dec 18, 2009, at 12:09 PM, Patai Gergely wrote: Hi, I would like to write a routine like nextPtr :: Storable a = Ptr a - Ptr a nextPtr = (`plusPtr` sizeOf (undefined :: a)) which increments a pointer by the correct amount, but GHC complains that the type variable a is ambiguous. Maybe Foreign.Marshal.Array.advancePtr is what you really need in this case. Gergely -- http://www.fastmail.fm - The professional email service ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
