[Haskell-cafe] "mapping" an Enumerator

[Michael Snoyman]

On Wed, Dec 21, 2011 at 12:59 PM, Kannan Goundan <kannan at cakoose.com> wrote:
> Michael Snoyman wrote:
>
>> On Wed, Dec 21, 2011 at 12:35 PM, Kannan Goundan <kannan at cakoose.com>
>> 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.)
>>>
>>>
>>> _______________________________________________ Haskell-Cafe mailing
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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