• 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