I noticed you already came up with one solution judging by your recent
Stack Overflow question, but I wanted to mention another solution: you
can transform any `Parser` using `zoom` and a lens. In this case you can
write:
zoom decoded (foldAll step begin done)
On 05/17/2014 04:28 AM, Reuben Thomas wrote:
I'm trying to learn how to parse files with Pipes, using Pipes.Aeson
and Pipes.Parse. I'm a bit bewildered by the library - it seems quite
complex, and I couldn't find too many examples of how it should be used.
For example:
I have a file containing several JSON objects all of the same type.
I'd like to read them into my program one-at-a-time and reduce them
down to some value.
From the documentation it looks like I should be using foldAll to do
this. foldAll returns a Parser.
However, the decode function in Pipes.Aeson has the type decode ::
(Monad m, FromJSON a) => Parser ByteString m (Either DecodingError a)
so it also returns a Parser.
I've written my own function that extracts a field from a parsed JSON
object, which has type :: Parser ByteString IO (Either DecodingError
Double), and a second function of type :: Parser Double IO Double (and
body foldAll max 0 id).
Is it possible to compose these into a single parser of type :: Parser
ByteString IO (Either DecodingError Double)? The intention being that
I extract the field from all the objects in the file, then find the
maximum.
Eventually I want to be able to run evalStateT on a Parser and a
Producer, but I'm having difficulties creating the Parser.
I'm sure this either has a really simple solution or I'm approaching
it in the wrong way. While I'm here - are there any good resources or
projects I can read through that have used the Pipes.Parse (and maybe
Pipes.Aeson) library? It'd be helpful to see what idiomatic usage of
the libraries looks like.
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