DiffArray deadlock

Lauri Alanko la at iki.fi
Thu Jun 3 20:34:38 EDT 2004

The following program gets stuck in ghci, and when compiled with ghc and
run, fails with "Fail: thread blocked indefinitely".

import Data.Array.Diff
main = print (a // [((a ! 0, 1))] ! 0)
    where a = array (0,0) [(0,0)] :: DiffArray Int Int

When DiffArray is replaced with Array, it just prints out "1" as it

Apparently there is some kind of a deadlock internally, with the update
operation holding the lock already before the lookup operation is

I think the solution is to evaluate all the indices in the argument to
replaceDiffArray _before_ obtaining the lock. Changing a single line in
Diff.hs gives a partial quick fix:

@@ -287 +287 @@
-a `replaceDiffArray` ies = do
+a `replaceDiffArray` ies = sum (map fst ies) `seq` do

However, in DiffUArray, when the underlying imperative array is unboxed,
the operation is strict also in the elements, and then (and _only_ then)
the elements, too, need to be evaluated before the MVar is taken, or
otherwise their evaluation may lead to a deadlock.

I'm not really sure what would be the neatest solution.

Lauri Alanko
la at iki.fi

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