[Haskell-cafe] carry "state" around ....

[Galchin, Vasili]

what does a datatype with no constructors mean?

E.g.

data RSAStruct
data EVP_PKEY
data EVP_CIPHER
data EVP_CIPHER_CTX
data EVP_MD_CTX
data EVP_MD
data BIGNUM



On Mon, Jul 28, 2008 at 8:01 PM, Felipe Lessa <felipe.lessa at gmail.com>wrote:

> 2008/7/28 Galchin, Vasili <vigalchin at gmail.com>:
> >> and we're suggesting instead:
> >>
> >> newtype AIOCB = AIOCB (ForeignPtr AIOCB)
> >
> >       ^^^ I am somewhat new to Haskell. Not a total newbie! But what
> exactly
> > does the above mean? Are the three references of "AIOCB" in different
> > namespaces?
>
> The first and the third are the type AIOCB, the second is the type
> constructor AIOCB. That is, it is equivalent (up to renaming) to
>
> newtype T = C (ForeignPtr T)
>
> Now, why use Type in Type's definition? It is obvious that if we were
> creating
>
> data T = D T
>
> it would be pretty useless, however the type that ForeignPtr requires
> is just a phantom type. In other words, the ForeignPtr will never use
> the C constructor.
>
> An analogy to C: if you have
>
> typeA *pa;
> typeB *pb;
>
> then of course pa and pb have different types, however their internal
> representation are the same: an integral type of 32/64 bits. The C
> compiler only uses the type to provide warnings, to know the fields'
> offsets, the size of the structure, etc. The same goes for Haskell, if
> you have
>
> pa :: ForeignPtr A
> pb :: ForeignPtr B
>
> then both pa and pb have different types, but again they have the same
> internal representation. However, for example, if you allocate memory
> for pa via Storable then the compiler will find the correct sizeOf
> definition because will gave the type hint. The compiler also won't
> you let mix pa and pb like in [pa,pb].
>
>
>
> So, if you declare
>
> newtype T = C (ForeignPtr T)
>
> you are:
>
> 1) Hiding the ForeignPtr from the users of your library if you don't export
> C.
> 2) Having type safeness by using ForeignPtr T instead of something
> generic like ForeignPtr () -- the same as using typeA* instead of
> void*.
> 3) Not needing to create a different type, like
>
> data InternalT = InternalT
> newtype T = C (ForeignPtr InternalT)
>
>
> Well.. did it help at all? =)
>
> --
> Felipe.
>
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