[MLton-user] Apparent bug in PackWord32Big.subVec

[Matthew Fluet]

Ryan Newton wrote:
> 
> The following code works in SML/NJ, but generates a type error in MLton.
> 
> fun read_int32 vec i =
>  Int32.fromLarge(Word32.toLargeInt(PackWord32Big.subVec(vec,0)))
> 
>> From the error message, it looks like PackWord32Big.subVec is
> returning a Word64.word!

The Basis Library specifies that in the PACK_WORD signature,
   val subVec : Word8Vector.vector * int -> LargeWord.word

In SML/NJ, LargeWord = Word32, but in MLton, LargeWord = Word64.

You should do the following:

fun read_int32 vec i =
   Int32.fromLarge (LargeWord.toLargeInt (PackWord32Big.subVec (vec, 0)))

and use the LargeWord structure to convert.