Elm Radio

040: Phantom Builder Pattern

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  • Phantom types (happens at compile time, not runtime)

  • Helps avoid things like adding centimeters and inches

  • ianmackenzie/elm-units

  • Joël Quenneville's phantom types talk from Elm in the Spring A Number by Any Other Name

  • Extensible records

  • Builder pattern

  • Jeroen's Hierarchy of Constraints

  • elm-graphql SelectionSet scope type variable

  • Builder Pattern episode

  • Brian Hicks' builder pattern talk Robot Buttons from Mars

  • with functions

  • Phantom builder is a state machine for your types

  • elm-review Rule API uses phantom builder

  • Phantom types discourse post on time package Simon Herteby

  • Snapshot test in Elm review for expected error messages

  • Phantom Builder live stream episode

Possible operations with phantom extensible builders
  • Add a new field
  • Remove a field
  • Change the type of a field
  • Remove the previously existing phantom type and change it to an empty record (not extensible, just a hardcoded return type) i.e. Replace
What you can do with phantom builder
  • Require something to be always called
  • Forbid something being called more than once
  • Cause other constraints dynamically after calling something
  • Make function calls mutually exclusive
  • Enable a function only if another one has been called
...more
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Elm RadioBy Dillon Kearns, Jeroen Engels
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