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clarify that Decode.map2 takes a constructor function as first argument #243

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clarify that Decode.map2 takes a constructor function as first argument #243

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4 changes: 3 additions & 1 deletion book/effects/json.md
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Expand Up @@ -273,7 +273,7 @@ That is all we needed for our HTTP example, but decoders can do more! For exampl
map2 : (a -> b -> value) -> Decoder a -> Decoder b -> Decoder value
```

This function takes in two decoders. It tries them both and combines their results. So now we can put together two different decoders:
This function takes in a type constructor function and two decoders. It tries both decoders and combines their results using the type constructor. So now we can put together two different decoders:

```elm
import Json.Decode exposing (Decoder, map2, field, string, int)
Expand All @@ -290,6 +290,8 @@ personDecoder =
(field "age" int)
```

Here, since a type alias generates a constructor function for its type, the `Person` constructor is the first argument to map2, followed by the two decoders.

So if we used `personDecoder` on `{ "name": "Tom", "age": 42 }` we would get out an Elm value like `Person "Tom" 42`.

If we really wanted to get into the spirit of decoders, we would define `personDecoder` as `map2 Person nameDecoder ageDecoder` using our previous definitions. You always want to be building your decoders up from smaller building blocks!
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