Ever since I learned about Domain Driven Design from Scott Wlaschin, as he applies it to Type driven design in ML languages (specifically F#), and about Type narrowing from Richard Feldman in his talk, Scaling Elm Apps, I’ve wanted to learn more about combining these two approaches to form a first c…

[image] nickwalt: What I’m looking for in all this is to find an approach to domain modelling in Elm that, like DDD, places the domain very deliberately in the driver’s seat. I wonder if so-called Hexagonal Architecture could provide some inspiration here: Hexagonal architecture (s…

… but in that circumstance one would be better suited to have the DoodadLength be the input instead of the parent object which happens to have a length property. That would also be easier to justify given the semantic import of DoodadLength, and lack of semantic import of “some record that has a len…

Since we are talking about Doodads we are already getting far away from talking about much that is even sensible so we probably should not split hairs over this example. Richard gives some more realistic example of extensible records in his talk. What I was trying to point out is that records and c…

[image] nickwalt: [image] pd-andy: Extensible records do precisely nothing with regards to custom types. They are two unrelated concepts. Striving for a flat model seems incredibly antithetical to good data modelling. The world isn’t flat, after all. These two points illustrate the pro…

[image] nickwalt: Coming back to Extensible Records (Extensible Alias Types?), I’m still not sure about their use as a mainstream pattern to assist with narrowing types. Yes, I think maybe you somehow got the idea that extensible records should be used to narrow the inputs to functions, or na…

[image] nickwalt: Given our desire to design with TEA always in mind, as fundamental to data modelling in Elm In my own code, I don’t design Elm programs with TEA as the fundamental building block. Instead I design a series of domain types and functions (a coworker described this as the “libr…

[image] joelq: Game.tick diff model Game.move direction model Game.appendNewSection section model Game.start model Would I be right in guessing that these Game functions all operate over extensible records, that provide the appropriate sub-set of model fields that they need (and nothing more)…

[image] rupert: Would I be right in guessing that these Game functions all operate over extensible records, that provide the appropriate sub-set of model fields that they need (and nothing more)? @rupert @joelq I’m also very interested in the use of Extensible Records in this code. Most look…

Weird, I realise that I’ve been using Extensible Records as a way to update values in a record but not to narrow. type ButtonStatus = EnabledButton | DisabledButton type TaskStartTime = TaskStartTime Time.Posix type Msg = ButtonClickedMsg | TaskStartTimeMsg Time.Posix type al…

There are no extensible records in the example code you just posted. Only regular records. The record update syntax ({ model | field = value }) looks a lot like the extensible record syntax ({ a | field : Type }), but it operates on ordinary records. Edit: I suppose the type of { model | field = …

Domain Driven Type Narrowing

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nickwalt October 1, 2021, 11:52am 38

Good question - I cannot tell. I’ll have to learn all about the builder pattern in Elm before I can answer that question.

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Domain Driven Type Narrowing

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