Add hint about order sensitivity of map function (json decode)

[maxzirps]

Programming background: >2 years javascript Learning Elm to learn functional programming and its concepts I was confused by the fact that the map function of json decode is order sensitive,

so if my JSON response looks like this:

{"name": "Willy Wonka",
 "address": "Chocolate Factory"}

and I decode it with:

decoder: Decoder Person
decoder = 
  JD.map2 Question
    (field "address" string)
    (field "name" string)

the values are assigned to the wrong fields.

[teehemkay]

Isn't this explicit in map2 type signature: map2 : (a -> b -> result) -> Decoder a -> Decoder b -> Decoder result

Also remember that "the order of arguments in the record constructor match the order of fields in the type alias!" rather then the map2 function.

Hope this helps

[David-Klemenc]

@maxzirps It is not the JSON that must have the fields in order but rather the fact that you are using a Record Constructor:

-- you probably have a Question record

type alias Question = { address: String, name: String }

-- you can create a question with the constructor:

Question "Some Address" "A Name" -- here the order matters!

-- another example

type alias Person = { name: String, age: Int }

Person "Hercules" 41 -- creates a Person record

The main thing to notice is that in the decoder Question is used as a constructor (... a function). I believe this usage is a bit tricky to notice. If you wold decode the Question to a tuple it would look like so:

type alias Question = (String, String) -- defined as a tuple

decoder: Decoder Question
decoder = 
  JD.map2 Tuple.pair
    (field "address" string)
    (field "name" string)