Why does sizeOf Word64 = 4?

[John Meacham]

On Sat, Nov 08, 2003 at 06:32:35PM -0800, Ben Escoto wrote:
> Hi, I'm trying to learn about Haskell's FFI (running 6.0.1 on linux)
> and see the following weird behavior with ghci:
> 	Prelude> :module Data.Word Foreign.Storable Foreign.Ptr
> 	Prelude Foreign.Ptr Foreign.Storable Data.Word> sizeOf nullPtr
> 	4
> 	Prelude Foreign.Ptr Foreign.Storable Data.Word> sizeOf
> 	                                          (nullPtr :: Ptr Word64)
> 	4
> Shouldn't "sizeOf nullPtr" return an error?  And sizeOf a Ptr Word64
> should be 8 I think.  Also this program prints "4", when it seems it
> should print "8":
> 
> 	module Main where
> 	import Data.Word
> 	import Foreign.Storable
> 	import Foreign.Ptr
> 	import Foreign.StablePtr
> 
> 	x :: Word64
> 	x = 5
> 
> 	main = do x_sptr <- newStablePtr 5
> 	          putStrLn $ show $ sizeOf x_sptr
> 
> These are artificial examples, but I originally noticed this when
> trying to decode a C structure.  Is this a bug in ghc or am I
> misunderstanding what sizeOf is supposed to do?

sizeof returns the size of its argument, since the argument you passed
is a pointer, so it properly returns the size of a pointer, 32 bits or 4
bytes on your architecture.

to find the size of the actual datatype do something like
sizeOf (undefined :: Word16) => 2
sizeOf (undefined :: Word64) => 8

sizeOf never evaluates its arguments so it is okay to pass anything to
it of the type you are interested in.

        John


-- 
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John Meacham - California Institute of Technology, Alum. - john at foo.net
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