Hello everyone! I’m still a beginner in Elm but also in typed functional programming in general, so sorry in advance if this question sounds weird. I came across the Json.Decode.Decoder type while reading the Todo MVC’s source code , and I went to look at the type definition : {-| A value that knows…

The Decoder functionality is implemented in what is called Kernel code - the javascript underlying elm. The compiler cannot typecheck javascript code, but we still need types on the elm side. For instance, a few lines below that definition, we see string : Decoder String string = Elm.Kernel.Jso…

Thank you a lot, I didn’t know about Kernel code. If I may ask another question about types and values, when you have a lower case in the type it means it can hold anything, right? For example (from the Elm guides) type List a = Empty | Node a List. But in the decoder example it only appears on the…

Declaring a type like type Val a = Val is a fancy trick called a phantom type. We’re parameterizing the type even though we’re not using the parameter in the value. This allows us to do fancy things like: stringVal : Val String stringVal = Val boolVal : Val Bool boolVal = Val boolValsOnly : V…

BTW, if you’re interested in getting a conceptual understanding of what the Json.Decode.Decoder and Json.Decode.Value types mean under the hood, this series of articles walks through implementing your own in pure Elm. While fun and educational, it’s worth noting that you absolutely can master the c…

Thanks for the phantom types explanation and the articles. Really appreciated :smile:

I must link this fantastic article explaining how you could have created the Json.Decode library and the Decoder type step by step: [image] New blog post: You could have designed the Json.Decode library! Show and Tell At elm-conf I had a weird idea about writing a universa…

Understanding the Decoder type

Learn

mattpiz April 29, 2018, 4:37am 7

I must link this fantastic article explaining how you could have created the Json.Decode library and the Decoder type step by step:

Topic		Replies	Views
Understanding Decoder Learn	4	1863	October 16, 2019
Elm/json re-implemented in Elm for learning Show and Tell	6	1442	September 30, 2019
Elm Design Question: Do we need JSON decoders? Learn	13	3118	July 9, 2019
Inspiration: Cool decoder type from TypeScript Request Feedback	5	2073	August 21, 2019
Generic decoding of json to an elm record Learn	17	4112	June 13, 2019

Understanding the Decoder type

Related topics