ByteArray# as a foreign import argument?

[Csaba Hruska]

It's a primitive type.
https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/prelude/primops.txt.pp#L1388
https://gitlab.haskell.org/ghc/ghc/wikis/commentary/prim-ops

Cheers,
Csaba

On Thu, Oct 10, 2019 at 8:16 PM Shao, Cheng <cheng.shao at tweag.io> wrote:

> Hello devs,
>
> I've been trying to figure out how to pass lifted types as foreign
> types, then encountered the following code in the `DsCCall` module
> (
> https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/deSugar/DsCCall.hs#L172
> ):
>
> ```
>   -- Byte-arrays, both mutable and otherwise; hack warning
>   -- We're looking for values of type ByteArray, MutableByteArray
>   --    data ByteArray          ix = ByteArray        ix ix ByteArray#
>   --    data MutableByteArray s ix = MutableByteArray ix ix
> (MutableByteArray# s)
>   | is_product_type &&
>     data_con_arity == 3 &&
>     isJust maybe_arg3_tycon &&
>     (arg3_tycon ==  byteArrayPrimTyCon ||
>      arg3_tycon ==  mutableByteArrayPrimTyCon)
>   = do case_bndr <- newSysLocalDs arg_ty
>        vars@[_l_var, _r_var, arr_cts_var] <- newSysLocalsDs
> data_con_arg_tys
>        return (Var arr_cts_var,
>                \ body -> Case arg case_bndr (exprType body) [(DataAlt
> data_con,vars,body)]
>               )
> ```
>
> It seems we allow a "ByteArray" type as a foreign import argument, if
> the third field of the datacon is a ByteArray# or MutableByteArray#.
> But I can't find such a ByteArray type definition in today's common
> packages. What's the rationale for this piece of code?
>
> Cheers,
> Cheng
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