如何使用Text.JSON解析从Haskell中的文件读取的字符串

Question

I'm writing an application in Haskell in which as part of my persistence system, I am converting my data structures to JSON format and then writing them to disk. Later, I want to load them by reading the file and running it through a JSON parser to reconstitute the objects.

However, something goes wrong in the read portion that prevents the JSON module from parsing the string.

First, I am using Text.JSON (json-0.5) for my work. The datatype that I am writing to file is called the WorkoutEvent , and I have verified that it encodes and decodes to JSON perfectly using this command:

decode $ encode evt :: Result WorkoutEvent

So, with that in mind, I create a bunch of events and write them to a file, and the resulting file looks like this (when I open it in my text editor):

{"type":"AddWorkout","data":{"uuid":"473b7964-8197-49ac-be3b-2b3804f1fbb5","date":"2012-03-30","workout_type":"Pushups","description":"","sets":[6]}}
{"type":"AddWorkout","data":{"uuid":"9f83363a-5298-4778-9c80-7dfa0d2ea6f9","date":"2012-04-02","workout_type":"Pushups","description":"","sets":[6,6]}}
{"type":"AddWorkout","data":{"uuid":"a60806a0-efd6-4647-bc62-a48298ce55cd","date":"2012-04-04","workout_type":"Pushups","description":"","sets":[]}}

And so on and so forth. Each line in the file represents one full object that I have serialized.

Then I load up the file using readFile and I see no problems:

*Main> f <- readFile "/home/savanni/Documents/workouts.json-stream"
*Main> take 50 f
"{\"type\":\"AddWorkout\",\"data\":{\"uuid\":\"473b7964-8197"
*Main> putStrLn $ take 50 f
{"type":"AddWorkout","data":{"uuid":"473b7964-8197

So far, so good. The catch comes when I try to decode the data:

*Main> decode f :: Result [WorkoutEvent]
Error "Invalid tokens at end of JSON string: \"{\\\"type\\\":\\\"A\""

If I say putStrLn f I see the output and see that there are no \\ sequences anywhere in the output. If I just enter f at the command line, I see escape sequences as above, but I never see any multiple-\\ escape sequences anywhere.

So, in the end, how exactly am I supposed to read JSON data from a file using the Text.JSON module?

Answer 1

Each line is valid JSON, but the entire file isn't, so you need to decode it line by line. Something like (untested):

map (decode :: String -> Result WorkoutEvent) $ lines f