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

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

Michael Snoyman wrote:

...

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

...

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