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Reading and writing.

Primitive arrays are linear, and indexed starting at zero. The size and indices of a `ByteArray#', `Addr#', and `MutableByteArray#' are all in bytes. It's up to the program to calculate the correct byte offset from the start of the array. This allows a `ByteArray#' to contain a mixture of values of different type, which is often needed when preparing data for and unpicking results from C. (Umm... not true of indices... WDP 95/09) Should we provide some `sizeOfDouble#' constants? Out-of-range errors on indexing should be caught by the code which uses the primitive operation; the primitive operations themselves do not check for out-of-range indexes. The intention is that the primitive ops compile to one machine instruction or thereabouts. We use the terms "reading" and "writing" to refer to accessing mutable arrays (see Section See section Mutable arrays), and "indexing" to refer to reading a value from an immutable array. Immutable byte arrays are straightforward to index (all indices in bytes): 
indexCharArray#   :: ByteArray# -> Int# -> Char#
indexIntArray#    :: ByteArray# -> Int# -> Int#
indexAddrArray#   :: ByteArray# -> Int# -> Addr#
indexFloatArray#  :: ByteArray# -> Int# -> Float#
indexDoubleArray# :: ByteArray# -> Int# -> Double#

indexCharOffAddr#   :: Addr# -> Int# -> Char#
indexIntOffAddr#    :: Addr# -> Int# -> Int#
indexFloatOffAddr#  :: Addr# -> Int# -> Float#
indexDoubleOffAddr# :: Addr# -> Int# -> Double#
indexAddrOffAddr#   :: Addr# -> Int# -> Addr#   
 -- Get an Addr# from an Addr# offset
The last of these, `indexAddrOffAddr#', extracts an `Addr#' using an offset from another `Addr#', thereby providing the ability to follow a chain of C pointers. Something a bit more interesting goes on when indexing arrays of boxed objects, because the result is simply the boxed object. So presumably it should be entered -- we never usually return an unevaluated object! This is a pain: primitive ops aren't supposed to do complicated things like enter objects. The current solution is to return a lifted value, but I don't like it! 
indexArray#       :: Array# elt -> Int# -> PrelBase.Lift elt  -- Yuk!
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