[Haskell-cafe] Can I allow multiple rigid type variables?

[Cody Goodman]

Nevermind, my last message would only ever allow me to do self inner joins.

On Thu, Jan 12, 2017 at 11:55 PM, Cody Goodman <
codygman.consulting at gmail.com> wrote:

> 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 at 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/fr
>> ames-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)
>>
>
>
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