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-- | This is the default version of alterF that we use in most non-trivial |
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-- cases. It's called "eager" because it looks up the given key in the map |
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-- eagerly, whether or not the given function requires that information. |
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alterFEager :: (Functor f, Eq k, Hashable k) |
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=> (Maybe v -> f (Maybe v)) -> k -> HashMap k v -> f (HashMap k v) |
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alterFEager f !k !m = (<$> f mv) $ \fres -> |
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case fres of |
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------------------------------ |
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-- Delete the key from the map. |
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Nothing -> case lookupRes of |
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-- Key did not exist in the map to begin with, no-op |
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Absent -> m |
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-- Key did exist, no collision |
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Present _ collPos -> deleteKeyExists collPos h k m |
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------------------------------ |
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-- Update value |
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Just v' -> case lookupRes of |
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-- Key did not exist before, insert v' under a new key |
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Absent -> insertNewKey h k v' m |
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-- Key existed before, no hash collision |
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Present v collPos -> v' `seq` |
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if v `ptrEq` v' |
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-- If the value is identical, no-op |
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then m |
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-- If the value changed, update the value. |
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else insertKeyExists collPos h k v' m |
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where !h = hash k |
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!lookupRes = lookupRecordCollision h k m |
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!mv = case lookupRes of |
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Absent -> Nothing |
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Present v _ -> Just v |
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{-# INLINABLE alterFEager #-} |
The problem is in lines 404–407: v' is never forced.
I wonder whether confusion about the strictness of insertNewKey may have led to this bug. Although this function doesn't have a module prefix, it is imported from Data.HashMap.Internal. The code author may have thought that they were using a local, strict version of insertNewKey.
