John Meacham wrote:
So, I finally decided that jhc needs real arrays, but am running into an
issue and was wondering how other compilers solve it, or if there is a
general accepted way to do so.
here is what I have so far
-- The opaque internal array type
data Array__ a
-- the array transformer quasi-monad
newtype AT a = AT (Array__ -> Array__)
seqAT__ :: AT a -> AT a -> AT a
seqAT__ (AT a1) (AT a2) = AT $ \a -> a2 (a1 a)
doneAT__ :: AT a
doneAT__ = AT id
newAT__ :: Int -> AT a -> Array__ a
newAT__ n (AT a1) = a1 (prim_newAT__ n)
writeAT__ :: Int -> a -> AT a
writeAT__ i x = AT $ \a -> prim_writeAT__ i x a
-- none of these routines have run-time checks
foreign import primitive "prim_newAT__" :: Int -> Array__
-- performs *update-in-place*
foreign import primitive "prim_writeAT__" :: Int -> a -> Array__ -> Array__
foreign import primitive "unsafeAt__" :: Array__ a -> Int -> a
-- example use
newArray :: [a] -> Array__ a
newArray xs = newAT__ (length as) $ foldr assign doneAT (zip [0..] xs) where
assign (i,v) rs = writeAT__ i v `seqAT__` rs
now, the problem occurs in newAT__
newAT__ :: Int -> AT a -> Array__ a
newAT__ n (AT a1) = a1 (prim_newAT__ n)
^ this gets floated out as a CAF.
In GHC, the primitive is this:
newArray# :: Int# -> a -> State# s -> (# State# s, MutArr# s a #)
that is, it takes a state and returns a new state. In order for calls
to newArray# to not be shared more than we want, we have to make sure
that the state argument to newArray# is never a constant visible to the
compiler. This entails, as you guessed, not inlining the definition of
unsafePerformIO or runST. See comments near the definition of runST in
libraries/base/GHC/ST.lhs for a description of exactly the problem you
describe.
Cheers,
Simon
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