032: Elm's Universal Pattern

• Joël Quenneville (Twitter)
• Joël's blog post Elm's Universal Pattern
• map2
• Maybe.map2

Metaphors

Some common metaphors for Elm's Universal Pattern (Applicative Pattern).

• Mapping

• Combining

• Lifting

• Wrapping and unwrapping boxes

• Blog post on Two ways of looking at map functions

Examples

• Random generators
• Apply mapping functions to vanilla value functions to keep things clean

Tips

• Separate branching code from doing code (discussed in-depth in Joël's blog post Problem Solving with Maybe)

• Stay at one level of abstraction

• Json decoders as combining functions

• Scott Wlaschin Railway Oriented Programming

• Dillon's blog post Combinators - Inverting Top-Down Transforms

• The JSON structure and Elm type don't have to mirror each other - start with your ideal type and work backwards

• Applicative pattern

• Applicative needs 1) way to construct, 2) map2 or andMap

• Json.Decode.Pipeline.required function

Record constructors

• Practice writing it with an anonymous function to convince yourself it's equivalent
• Record constructors are just a plain old elm function
• map2 doesn't take a type, it takes a function -
• NoRedInk/elm-json-decode-pipeline is a useful reference for implementing this kind of api on your own
• Applicative Laws in Haskell
• Monomorphic vs polymorphic
• Parser Combinators
• elm/parser episode
• Joël's blog posts on the ThoughtBot blog
• Joël's Random generators talk
• Joël's Maybe talk

Some more blog posts by Joël that related to Elm's Universal Pattern:

• Running out of maps
• Pipeline Decoders in Elm

Joël's journey to building a parser combinator:

• Nested cases - https://ellie-app.com/b9nGmZVp9Vca1
• Extracted Result functions - https://ellie-app.com/b9qtqTf8zYda1
• Introducing a Parser alias and map2 - https://ellie-app.com/b9MwZ3y4t8ra1
• Re-implementing with elm/parser - https://ellie-app.com/b9NZhkTGdfya1
• Getting Unstuck with Elm JSON Decoders - because mapping is universal, you can solve equivalent problems with the same pattern (described in this post)