Re: [Haskell-cafe] "mapping" an Enumerator

Michael Snoyman wrote:

On Wed, Dec 21, 2011 at 12:59 PM, Kannan Goundan wrote:

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Michael Snoyman wrote:

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On Wed, Dec 21, 2011 at 12:35 PM, Kannan Goundan wrote:

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I'm using the Data.Enumerator library.  I'm trying to write a "map" function that converts an Enumerator of one type to another. Something like:

  mapEnum :: Monad m =>          (a -> b) ->          Enumerator a m r ->          Enumerator b m r

Any hints?

(My exact use case is that I have a ByteString enumerator and I need to pass it to something that requires a Blaze.Builder enumerator.)

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You can use the Data.Enumerator.List.map function to create an Enumeratee, and then the Data.Enumerator.$= operators to join them together. Something like:

mapEnum f enum = enum $= EL.map f

I tried something like that but the resulting type isn't quite what I'm looking for.

  mapEnum :: Monad m =>         (a -> b) ->         Enumerator a m (Step b m r) ->         Enumerator a m r

(BTW, Michael, my exact use case is that I have ByteString enumerators, but your HTTP-Enumerator library wants Blaze.Builder enumerators :-)

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Huh, I'm stumped ;). John: is this possible in enumerator?

In general though: do you need precisely that type signature? Most of the time, Enumerators have polymorphic return types. It might be a problem from http-enumerator requiring (), but I *think* we can work around that.

Michael

You're right -- now that I think about it, I don't really care what the type of 'r' is. (Boy, I could have saved several hours today if I had realized that earlier :-) But yeah, I'm having trouble matching the () in http-enumerator's interface. (I wonder if it's because I'm trying to use a fully concrete helper type 'MyRequestBody'): data MyRequestBody = MyRequestBody Int64 (Enumerator ByteString IO ()) myHttp :: URL -> MyRequestBody -> IO () myHttp url (MyRequestBody len bsEnum) = ... where builderEnum = mapEnum BlazeBS.fromByteString bsEnum reqBody = RequestBodyEnum len builderEnum GHC tells me that mapEnum expects bsEnum to be: Enumerator ByteString IO (Step Blaze.Builder IO ()) But it is actually: Enumerator ByteString IO () Wonder if I can use a "forall" in the definition of MyRequestBody to fix things without introducing an externally visible type parameter... Also, thanks for your help so far!