How to Encode a dict?

Request Feedback
1

I’m trying to Encode a model that is using a dict attribute but I don’t know how to encode that parte of the model, I’m trying to do the following.

type alias TemplateResponses =
{ userId : String
, name : String
, testResponses : List TestResponse
}

type alias TestResponse =
{ name : String
, responses : List Response
}

type alias Response =
{ id : String
, response : Dict Int String
}

emptyTemplateResponse : TemplateResponses
emptyTemplateResponse =
{ userId = “234”
, name = “”
, testResponses = []
}

templateResponseEncoder : TemplateResponses -> Encode.Value
templateResponseEncoder templateResponse =
Encode.object
[ ( “userId”, Encode.string templateResponse.userId )
, ( “uid”, Encode.string templateResponse.name )
, ( “testResponses”, Encode.list testResponseEncoder templateResponse.testResponses )
]

testResponseEncoder : TestResponse -> Encode.Value
testResponseEncoder testResponse =
Encode.object
[ ( “name”, Encode.string testResponse.name )
, ( “responses”, Encode.list responseEncoder testResponse.responses )
]

responseEncoder : Response -> Encode.Value
responseEncoder response =
Encode.object
[ ( “id”, Encode.string response.id )
, ( “response”, Encode.dict response.response)
]

but i receive the next error
The 2nd element is a tuple of type:
( String, #(v -> Encode.Value) -> Dict k v -> Encode.Value# )
But all the previous elements in the list are:
( String, #Encode.Value# )

2

You need to pass 3 arguments to Encode.dict, the 1st is a function to convert the key to string, the 2nd is the value encoder, and the third is the dict.
https://package.elm-lang.org/packages/elm/json/latest/Json-Encode#dict

3

Thanks for your commend, but I really don’t get it
I wrote the next code and It seems like everything is ok :

responseEncoder : Response -> Encode.Value
responseEncoder response =
Encode.object
[ ( “id”, Encode.string response.id )
, ( “response”, Encode.dict identity Encode.string response.response)
]

but what does mean identity?

4

identity is a function defined as identity a = a. It basically just returns its argument.

This is needed because Dict in elm can have comparable keys BUT the encoded dictionary keys must be String (you can’t have a javascript object field name be a tuple of Int). In your case, the keys are already String so, you don’t need to transform them so you use identity to just pass them through.

5

Actually my original key was an Int but I changed it to String because that was the only way to remove the error, but if I wanted to use an Int key I need to make a function that receipted all my keys in Int and return them in String?

6

You use String.fromInt instead of identity and you should be OK.

1 Like
7

ohhh I see, you’re totally right. thanks man, have a nice day.

8

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