040: Phantom Builder Pattern

• 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