040: Phantom Builder Pattern
Jeroen introduces the phantom builder pattern and how it enables new guarantees in Elm API design.
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Show Notes
Phantom types (happens at compile time, not runtime)
Helps avoid things like adding centimeters and inches
Joël Quenneville's phantom types talk from Elm in the Spring A Number by Any Other Name
Extensible records
Builder pattern
elm-graphqlSelectionSetscope type variableBrian Hicks' builder pattern talk Robot Buttons from Mars
withfunctionsPhantom builder is a state machine for your types
elm-reviewRule API uses phantom builderPhantom types discourse post on time package Simon Herteby
Snapshot test in Elm review for expected error messages
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