ByteArray# as a foreign import argument?

[Sylvain Henry]

 > But I can't find such a ByteArray type definition in today's common 
packages. What's the rationale for this piece of code?

Doing some archeology they seem to have been removed from 
ghc/lib/std/PrelArr.lhs in e921b2e307532e0f30eefa88b11a124be592bde4 (1999):

  data Ix ix => Array ix elt        = Array            ix ix (Array# elt)
-data Ix ix => ByteArray ix          = ByteArray ix ix ByteArray#
  data Ix ix => MutableArray     s ix elt = MutableArray     ix ix 
(MutableArray# s elt)
-data Ix ix => MutableByteArray s ix     = MutableByteArray ix ix 
(MutableByteArray# s)

So it's probably dead code since then.

Cheers,
Sylvain


On 10/10/2019 21:15, Shao, Cheng 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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