Johan Tibell wrote:
> On Tue, Feb 1, 2011 at 4:24 PM, Roman Leshchinskiy <[email protected]>
> wrote:
>
>> Johan Tibell wrote:
>>
>>>
>>> Does cloneArray# really have to take and return a `State# s`?
>>>
>>
>> Oops, no, it doesn't. This was purely a case of mindless copy-pasting.
>>
>
> Does this apply to any of the other primops?
I don't think so. Anything involving allocating/reading/writing
MutableArray# is impure.
> thawArray# :: Array# a -> Int# -> Int# -> State# s -> (#
State# s, MutableArray# s a #)
>
>
> It seems to me that maybe thawArray# doesn't need to take a state
> token either as the input array can be copied at any point in time.
No, the operation is still impure. Suppose we had
thawArray# :: Array# a -> Int# -> Int# -> MutableArray# s a
Then, we write
f xs = let ys = thawArray# xs 0 n#
zs = thawArray# xs 0 n#
in e
Do ys and zs refer to the same MutableArray# in e?
> Is it
> even possible to return a MutableArray# without taking a state token as an
> input parameter?
No, for precisely this reason. In contrast to pure arrays, mutable arrays
have an "identity", hence allocating them is impure.
Roman
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