You're welcome!
This is more odd than useful, but these "fallible folds" admit a monad
instance. If you wrap them in a newtype
newtype Fallible m r i e = Fallible { getFallible :: FoldM (ExceptT e m) i
r }
Then you can define *return *as "create a fold that begins in a failed
state" and *>>= *as "if an error is encountered, keep folding using a new
fold constructed from the error".
On Friday, October 2, 2015 at 10:23:39 PM UTC+2, Dylan Tisdall wrote:
>
> Thanks, Daniel, that was a really helpful suggestion! In case someone else
> is ever looking for an example of using ExceptT and Control.Foldl to
> validate/process data from a Producer, stopping immediately if any of the
> operations experience an error, here's a small program demonstrating how to
> do it:
>
> http://pastebin.com/HgXQRm3p
>
> This example produces a list of pairs, and then folds over them to
> validate some ordering properties of the pairs, and also prints the pairs
> out as they're produced. Running the example shows that if any of the
> validators experience an error, the computation is terminated and we get
> the error back immediately.
>
>
> Thanks again,
> Dylan
>
> On Friday, October 2, 2015 at 10:31:18 AM UTC-4, Daniel Díaz wrote:
>
> > What's unclear to me is how to make all of the combined `Fold`s stop if
> any one of them encounter an error state.
>
> You could try using a monadic fold that works in the ExceptT
> <http://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Monad-Trans-Except.html>
>
> monad transformer. Such folds can "stop early".
>
> On Friday, October 2, 2015 at 3:47:00 PM UTC+2, Dylan Tisdall wrote:
>
> Maybe I should ask a more general question, in case I'm going about this
> all wrong with my previous detailed question. I'm processing a file that
> consists of millions of "records". The records (`MDHAndScanLine`) were
> generated by a process with a series of nested loops, so each record
> contains data, and the loop counters when the data was generated. Having
> parsed these records into a producer of type `P.Producer MDHAndScanLine m
> (Either String ())`, I've basically flattened out all the loops into a
> single list. My problem is now that I've got multiple operations I'd like
> to do in one pass down the `Producer` (e.g., scan through it and make sure
> that the loop counters are all in the order I expect; do processing of type
> A to the data and output the result; do processing of type B on only data
> with certain loop counters and output the result). Ideally, if any of these
> operations encounter a failure, I'd like to report the error and bail on
> the whole thing.
>
> I was originally thinking of making each one of these operations a Pipe,
> and having them stop `yield`-ing and return errors immediately if something
> goes wrong; that's what lead to my questions about FreeT. However, based on
> the ongoing thread "Help folding a Producer into a value", I'm getting
> the sense that Control.Foldl might be the better tool for my problem. I can
> see that if I write each of my operations at a `Fold` (or `FoldM` for
> logging output along the way), then I can combine them all into one joint
> operation that will traverse the `Producer`. I can see that you can use the
> fold's state to keep track of whether you've encountered an error, and then
> the fold's step function can just stop updating on new data. What's unclear
> to me is how to make all of the combined `Fold`s stop if any one of them
> encounter an error state.
>
> Also, the nice part of the Pipe-style solution I described first is that I
> don't think it'll keep reading the file once it encounters an error (it
> will stop `await`-ing one it returns an error). My impression is that the
> Fold approach will insist on traversing the whole file every time, and just
> ignore all the data once its had an error. Or perhaps I've totally missed
> something about Fold that allows it to stop early if it has its result.
>
> Sorry for the confusion! These tools all seem really helpful, but I'm
> still trying to figure out how to put them together the right way.
>
>
> Thanks again,
> Dylan
>
> On Thursday, October 1, 2015 at 5:11:12 PM UTC-4, Dylan Tisdall wrote:
>
> Thanks Michael and Gabriel for your help; despite appearances here, I am
> actually making headway on my project!
>
> I've run into a new point of confusion, though, related to how to allow
> failure in parsing over groups. More explicitly:
>
> My data naturally breaks into groups, so I've got a method
>
> groupByID :: Monad m => P.Producer MDHAndScanLine m (Either String ()) ->
> P.FreeT (P.Producer MDHAndScanLine m) m (Either String ())
>
> That takes my Producer of MDHAndScanLines and breaks them using the
> relevant comparison. My question is now how to parse this new FreeT
> structure when it's possible that the parsing might fail. For example, I
> want to process each group, making them into a new data type `GroupedMDHs`,
> but it's possible I'll discover as I'm consuming the data that my
> preconditions for making a `GroupedMDHs` are violated, at which point I
> want to fail. So I want a method like:
>
> example: :: Monad m => P.FreeT (P.Producer MDHAndScanLine m) m (Either
> String ()) ->
> P.Producer GroupedMDHs m (Either String ())
>
> where I return `Left "Error"` if either the underlying Producer returns a
> `Left`, or if I discover I can't make a GroupedMDH.
>
> To make my example more concrete, say I have a function:
>
> groupedMDHParser :: Monad m => Int -> P.Parser MDHAndScanLine m (Either (
> Either String ()) GroupedMDHs)
>
> This parser consumes all the input from a producer, and makes a
> `GroupedMDHs`. However, if the length of data consumed doesn't match the
> first `Int` argument, then it returns `Left $ Left "Error"` and otherwise
> it returns a `GroupedMDHs`. I chose the type signature to be compatible
> with `parseForever`, since that would turn this parser into a pipe that
> matched my original producer type `P.Producer MDHAndScanLine m (Either
> String ())`. I've tried to find a way to apply this pipe to each element
> of the FreeT "list" in turn, but the return types don't match up. I
> understand at a conceptual level that I can't just map my transformation
> over each element of the FreeT, because if the application of `
> groupedMDHParser` fails, its implicitly also truncating the FreeT
> structure at that point and returning early. So this doesn't work:
>
> example groupedMDHScanLines = P.concats $ P.maps (\p -> p >->
> groupedMDHParser) groupedMDHScanLines
>
>
> I can also see that Pipes.Groups.folds is a candidate, but I don't see how
> I can get it to exit early if processing one of the groups fails. Is there
> a standard idiom for that kind of behaviour?
>
>
> Thanks again,
> Dylan
>
> On Sunday, September 27, 2015 at 12:05:52 PM UTC-4, Gabriel Gonzalez wrote:
>
> Like Michael mentioned, you want `Pipes.Prelude.fold'`
>
> If you know that your `FreeT` list only has one `Producer`, then you
> should encode that in the type by keeping it as a `Producer`. Then the
> question becomes how to fold that `Producer` directly instead of folding it
> within the context of a `FreeT` list, and that's what the `fold'` function
> does: it folds the producer and also preserves the return value:
>
> fold' :: Monad m => (x -> a -> x) -> x -> (x -> b) -> Producer a m r
> -> m (b, r)
>
> ... or combined with the `foldl` library it would be:
>
> purely fold' :: Monad m => Fold a b -> Producer a m r -> m (b, r)
>
> On 9/24/2015 8:47 PM, Dylan Tisdall wrote:
>
> I have a quick follow-up question, actually; pipes-group defines:
>
> Pipes.Group.folds :: Monad m => (x -> a -> x) -- ^ Step function
> -> x -- ^ Initial accumulator -> (x -> b) -- ^ Extraction function
> -> FreeT (Producer a m) m r -- ^ -> Producer b m r
>
> If I'm reading this right, when my FreeT "list" consists of just one
> Producer, then Pipes.Groups.folds returns a Producer that yields one
> output, and preserves the original Producer's return type, r, in the
> returned Producer. This is in contrast to the similar function
>
> Pipes.Prelude.fold :: Monad m => (x -> a -> x) -> x -> (x -> b) -> Producer a
> m () -> m b
>
> which only works on Producers with return type (). You note in the
> documentation for Pipes.Prelude.fold that this type is required because it
> may stop drawing from the Producer early, so you don't necessarily get to
> compute the return type. I'm wondering if it's easy to define a function
>
> foldToProducer :: Monad m => (x -> a -> x) -> x -> (x -> b) -> Producer a
> m r -> Producer b m r
>
> that does what I think Pipes.Group.folds is doing, but without needing
> all the FreeT bits as well. As an exercise, I tried to write foldToProducer,
> but couldn't figure it out.
>
>
> Thanks again,
> Dylan
>
>
> On Thursday, September 24, 2015 at 11:20:06 PM UTC-4, Dylan Tisdall wrote:
>
> Right, I wasn't recognizing that `Producer` was an instance of `Functor`
> since it's an instance of `Monad`, so I wasn't even looking there. Thanks
> again for all your help!
>
> On Tuesday, September 22, 2015 at 6:56:49 PM UTC-4, Gabriel Gonzalez
> wrote:
>
> Use the `void` function from `Control.Monad` if you want to erase the
> return type of a `Producer`:
>
> void :: Functor f => f a -> f ()
> void = fmap (\_ -> ())
>
> I might even re-export this from `pipes` as a convenience since this
> question comes up a lot.
>
> Originally functions like `Pipes.Prelude.length` had a more general type
> like this:
>
> Pipes.Prelude.length :: Producer a m r -> m Int
>
> ... but then at the advice of others I restricted the type to this:
>
> Pipes.Prelude.length :: Producer a m () -> m Int
>
> ... so that the user would have to explicitly discard the return value to
> signal that they were okay with ignoring that data. This is similar in
> principle to the warning you get if you turn on the `-Wall` flag that
> (among other things) warns if you have an unused non-empty return value,
> like this:
>
> example = do
> getLine // Compiler warning because you didn't use the result
> ...
>
> ... and you usually have to explicitly ignore the value using something
> like this syntax to indicate that you are intentionally ignoring the value:
>
> example = do
> _ <- getLine
> ...
>
> So the requirement to explicitly discard the value using `void` is in the
> same spirit as that compiler warning.
>
> On 9/22/15 3:50 PM, Dylan Tisdall wrote:
>
> Hi Gabriel,
>
> Thanks again for your help. That really clarified that I should be using
> lift to keep everything inside the Producer transfomer. To make all the
> types work, I ended up with:
>
> type MDHAndScanLineProducer = P.Producer MDHAndScanLine IO (Either
> (P.DecodingError, P.Producer P.ByteString IO ()) ())
>
> measDatMDHScanLinePairs :: Handle -> MDHAndScanLineProducer
> measDatMDHScanLinePairs h = do
> (hLen, leftovers) <- lift $ P.runStateT (P.decodeGet getWord32le) p
> case (hLen :: Either P.DecodingError Word32) of
> Left err -> return $ Left (err, leftovers)
> Right len -> do
> lift (hSeek h AbsoluteSeek (fromIntegral len))
> view P.decoded p
> where
> p = PB.fromHandle h
>
> This seems to work exactly as I'd hoped.
>
> As a follow-up, I'm now wondering how to use this producer and ignore its
> return type; effectively how to turn it into a Producer MDHAndScanLine IO
> (). This seems to be necessary to access many library functions. For
> example, I can't use
>
> Pipes.Prelude.length :: Monad m => Producer a m () -> m Int
>
> directly on the output of measDatMDHScanLinePairs because the return type
> doesn't match.
>
> Thanks again for all your help as I get up to speed on this!
>
>
> Dylan
>
>
> On Monday, September 21, 2015 at 11:43:58 PM UTC-4, Gabriel Gonzalez
> wrote:
>
> You're definitely on the right track. The type I would aim for would be
> something like this:
>
> example :: Handle -> Producer MDHAndScanLine IO (Either DecodingError
> (Producer ByteString IO ()))
>
> Notice that this slightly differs from your type; I'm merging the outer
> `IO (Either DecodingError ...)` into the first `Producer` to simplify the
> type.
>
> The implementation for that type would be very similar to the one you
> wrote in your second e-mail:
>
> example :: Handle -> Producer MDHAndScanLine IO (Either DecodingError
> (Producer ByteString IO ()))
> example handle = do
> let p = Pipes.ByteString.fromHandle handle
> x <- lift (evalStateT (decodeGet getWord32le) p)
> case x of
> Left err -> return (Left err)
> Right len -> do
> lift (hSeek handle AbsoluteSeek (fromIntegral l))
> view decoded p
>
> That will definitely run in constant memory, meaning that it won't ever
> load more than one chunk of bytes at a time (where a chunk is something
> like 32 kB, I think). You can profile the heap if you want to verify this
> by following these instructions:
>
>
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/prof-heap.html
>
> Also, to answer your other question, `pipes-attoparsec` runs in constant
> memory. The difference between `pipes-attoparsec` and `attoparsec` is that
> `pipes-attoparsec` runs a separate parser for each element in the stream,
> which is equivalent to "committing" after each parsed element. That means
> that it can only backtrack while parsing a single element in the stream,
> but no further back. This is why `pipes-attoparsec` runs in constant space
> over a large file and why `attoparsec` does not, because `attoparsec`
> backtracks indefinitely and `pipes-attoparsec` does not.
>
> On 9/21/15 12:10 PM, Dylan Tisdall wrote:
>
> Following up on my last question, my next issue is also probably a very
> straight ahead example of pipes, but I've managed to get tangled up going
> back and forth in the packages' documentation.
>
> I've got a file whose first 4 bytes give the offset into the file of a
> series of binary data elements (called MDHs in my case). Given a Handle to
> the start of such a file, I want to:
>
> 1. read the first Word32 in the file, to retrieve the offset;
> 2. skip the Handle to that offset; and
> 3. turn the rest of the file into a Producer MDH IO ()
>
> Given that the file I'm reading may be large, I want to make sure this
> process is going to run in constant memory. I thought I could use
> pipes-attoparsec, but I couldn't get straight whether it would need to read
> the whole file before it could produce anything (as I understand is
> normally the case with attoparsec).
>
> So far I have the following, which isn't complete, but at least does the
> skip and converts the remaining file to a ByteString producer.
>
> handleToMDHs :: Handle -> IO (Either P.DecodingError (P.Producer P.
> ByteString IO ()))
>
> ...
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