type vs data, again...

17 Mar 2013

      Hello,

 I'm again a bit stuck on a simple problem, and I think it's the same
problem that I faced some time ago:
http://comments.gmane.org/gmane.comp.lang.haskell.beginners/10843

 There is obviously something that I didn't get yet about "data" versus
"type".

 In the program in attachment, I want to parse a JSON file to a Value, then
if that value is in fact a JSON object, I want to explore its hash member
name -> member value.

 Now we have:

data Value = Object Object <--- the case I hope for           | Array
Array           | String Text           | Number Number           |
Bool !Bool           | Null             deriving (Eq, Show, Typeable,
Data)

And then:

-- | A JSON \"object\" (key\/value map).type Object = Map Text Value

So my clearly flawed plan is to get the value, pattern mach it against
(Object hash) and then work on the hash. However for the program in
attachment, which I would expect to compile, I get this compile error:

question.hs:12:62:
    Couldn't match expected type `Map.Map T.Text Value'
                with actual type `Object'
    In the first argument of `parseConfigMap', namely `map'
    In the second argument of `($)', namely `parseConfigMap map'
    In the expression: return $ parseConfigMap map

That confuses me, because if the error means by Object the data, then I
understood nothing. However if the error means by Object the type, then I
would expect it to work, since the type exactly means "Map Text Value"...

Any hint? And that really doesn't look like a good idea to me to define a
type and a data by the same name, but it's apparently the right way to do
it...

Thank you!

Emmanuel

Emmanuel Touzery

Andres Löh

Emmanuel Touzery

Andres Löh

Emmanuel Touzery

David McBride

Emmanuel Touzery

Rustom Mody

Gabriel Gonzalez

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participants (5)