I figured out a way to kind of sideskirt the problem where I define my main join as a helper and then dispatch on length in the calling function. GHC generated this type for it: innerJoin :: ((as ++ bs) ~ (bs ++ as), PrimMonad m, Ord k, RecVec (bs ++ as)) => Producer (Rec Identity bs) m () -> Getting k (Rec Identity bs) k -> Producer (Rec Identity as) m () -> Getting k (Rec Identity as) k -> m (FrameRec (bs ++ as)) What exactly does (as ++ bs) ~ (bs ++ as) mean? On Thu, Jan 12, 2017 at 11:48 PM, Cody Goodman < codygman.consulting@gmail.com> wrote:
I know my question doesn't quite make sense and you can't have "multiple rigid type variables" but I couldn't think of a better way to characterize it or explain it. So I'll also try to give more detail below along with the code example and error at the end.
I have an inner join function using vinyl which does cool stuff like giving a type error if you try to join on a column that doesn't exist. I previously had an implementation that blindly built an index based on the left argument.
I then tried to dispatch based on the lengths of the left and right producer arguments. The problem is my return type is:
Rec f (as ++ bs)
That means I have two different producers, one of as and one of bs. I then do a case statement on the lengths and I want to store the producer to use for a given piece in that variable. The problem is ghc "pins" the type variable as either (Rec f as) or (Rec f bs) when I need to make that choice at runtime.
My code and error is both at github here: https://github.com/codygman/ frames-diff/blob/inner-join-fix/Frames/Diff.hs#L113
and below:
-- | Performs an inner join and keeps any duplicate column -- Recommend keeping columns in producers disjoint because accessing -- anything but the leftmost duplicate column could prove difficult. -- see: https://github.com/VinylRecords/Vinyl/issues/55# issuecomment-269891633 innerJoin :: (MonadIO m, Ord k) => Producer (Rec f leftRows) IO () -- leftProducer -> Getting k (Rec f leftRows) k -- leftProducer lens -> Producer (Rec f rightRows) IO () -- rightProducer -> Getting k (Rec f rightRows) k -- rightProducer lens -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) innerJoin leftProducer leftLens rightProducer rightLens = do leftProducerLen <- P.liftIO $ P.length leftProducer rightProducerLen <- P.liftIO $ P.length rightProducer let curProducer = case rightProducerLen < leftProducerLen of True -> rightProducer -- False -> leftProducer let curKeymapProducer = case rightProducerLen < leftProducerLen of True -> leftProducer -- False -> rightProducer let curLensLookup = case rightProducerLen < leftProducerLen of True -> rightLens -- False -> leftLens let curLensInsert = case rightProducerLen < leftProducerLen of True -> leftLens -- False -> rightLens let appender km row = case rightProducerLen < leftProducerLen of True -> case M.lookup (view curLensLookup row) km of Just otherRow -> pure $ rappend otherRow row Nothing -> P.mzero -- False -> case M.lookup (view curLensLookup row) km of -- Just otherRow -> pure $ rappend row otherRow -- Nothing -> P.mzero keyMap <- P.liftIO $ P.fold (\m r -> M.insert (view curLensInsert r) r m) M.empty id curKeymapProducer pure $ curProducer >-> P.mapM (\r -> appender keyMap r) -- error if I uncomment my false cases (specifically the False case for curProducer) -- [5 of 5] Compiling Frames.Diff ( Frames/Diff.hs, interpreted ) -- Frames/Diff.hs:125:32: error: -- • Couldn't match type ‘leftRows’ with ‘rightRows’ -- ‘leftRows’ is a rigid type variable bound by -- the type signature for: -- innerJoin :: forall (m :: * -> *) k (f :: * -- -> *) (leftRows :: [*]) (rightRows :: [*]). -- (MonadIO m, Ord k) => -- Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- at Frames/Diff.hs:113:14 -- ‘rightRows’ is a rigid type variable bound by -- the type signature for: -- innerJoin :: forall (m :: * -> *) k (f :: * -- -> *) (leftRows :: [*]) (rightRows :: [*]). -- (MonadIO m, Ord k) => -- Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- at Frames/Diff.hs:113:14 -- Expected type: Producer (Rec f rightRows) IO () -- Actual type: Producer (Rec f leftRows) IO () -- • In the expression: leftProducer -- In a case alternative: False -> leftProducer -- In the expression: -- case rightProducerLen < leftProducerLen of { -- True -> rightProducer -- False -> leftProducer } -- • Relevant bindings include -- curProducer :: Producer (Rec f rightRows) IO () -- (bound at Frames/Diff.hs:123:7) -- rightLens :: Getting k (Rec f rightRows) k -- (bound at Frames/Diff.hs:119:47) -- rightProducer :: Producer (Rec f rightRows) IO () -- (bound at Frames/Diff.hs:119:33) -- leftLens :: Getting k (Rec f leftRows) k -- (bound at Frames/Diff.hs:119:24) -- leftProducer :: Producer (Rec f leftRows) IO () -- (bound at Frames/Diff.hs:119:11) -- innerJoin :: Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- (bound at Frames/Diff.hs:119:1)