Example of how to parse exiftool JSON output in Haskell
Question
I can't make sense of any of the documentation. Can someone please provide an example of how I can parse the following shortened exiftool
output using the Haskell module Text.JSON
? The data is generating using the command exiftool -G -j <files.jpg>
.
[{
"SourceFile": "DSC00690.JPG",
"ExifTool:ExifToolVersion": 7.82,
"File:FileName": "DSC00690.JPG",
"Composite:LightValue": 11.6
},
{
"SourceFile": "DSC00693.JPG",
"ExifTool:ExifToolVersion": 7.82,
"File:FileName": "DSC00693.JPG",
"EXIF:Compression": "JPEG (old-style)",
"EXIF:ThumbnailLength": 4817,
"Composite:LightValue": 13.0
},
{
"SourceFile": "DSC00694.JPG",
"ExifTool:ExifToolVersion": 7.82,
"File:FileName": "DSC00694.JPG",
"Composite:LightValue": 3.7
}]
Solution
Well, the easiest way is to get back a JSValue from the json package, like so (assuming your data is in text.json):
Prelude Text.JSON> s <- readFile "test.json"
Prelude Text.JSON> decode s :: Result JSValue
Ok (JSArray [JSObject (JSONObject {fromJSObject = [("SourceFile",JSString (JSONString {fromJSString = "DSC00690.JPG"})),("ExifTool:ExifToolVersion",JSRational False (391 % 50)),("File:FileName",JSString (JSONString {fromJSString = "DSC00690.JPG"})),("Composite:LightValue",JSRational False (58 % 5))]}),JSObject (JSONObject {fromJSObject = [("SourceFile",JSString (JSONString {fromJSString = "DSC00693.JPG"})),("ExifTool:ExifToolVersion",JSRational False (391 % 50)),("File:FileName",JSString (JSONString {fromJSString = "DSC00693.JPG"})),("EXIF:Compression",JSString (JSONString {fromJSString = "JPEG (old-style)"})),("EXIF:ThumbnailLength",JSRational False (4817 % 1)),("Composite:LightValue",JSRational False (13 % 1))]}),JSObject (JSONObject {fromJSObject = [("SourceFile",JSString (JSONString {fromJSString = "DSC00694.JPG"})),("ExifTool:ExifToolVersion",JSRational False (391 % 50)),("File:FileName",JSString (JSONString {fromJSString = "DSC00694.JPG"})),("Composite:LightValue",JSRational False (37 % 10))]})])
this just gives you a generic json Haskell data type.
The next step will be to define a custom Haskell data type for your data, and write an instance of JSON for that, that converts between JSValue's as above, and your type.
OTHER TIPS
Thanks to all. From your suggestions I was able to put together the following which translates the JSON back into name-value pairs.
data Exif =
Exif [(String, String)]
deriving (Eq, Ord, Show)
instance JSON Exif where
showJSON (Exif xs) = showJSONs xs
readJSON (JSObject obj) = Ok $ Exif [(n, s v) | (n, JSString v) <- o]
where
o = fromJSObject obj
s = fromJSString
Unfortunately, it seems the library is unable to translate the JSON straight back into a simple Haskell data structure. In Python, it is a one-liner: json.loads(s)
.