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)