F# Phantom Typing

Jul 24, 2023

The idea is that you will have a generic wrapper Event<'T> and use different types for 'T to represent different types of Events. These types (referring to 'T) are never actually instantiated, which is why they’re called phantom types.

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[<Struct>]
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type Event<'T> = Event of string
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type ClickEvent = interface end
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type KeyEvent = interface end
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// Now we can create events of different types using the phantom types:
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let createClickEvent (data: string) : Event<ClickEvent> = Event(data)
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let createKeyEvent (data: string) : Event<KeyEvent> = Event(data)
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// The nice thing about this is that we can write functions that work
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// with any event easily:
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let logEvent (Event data) = printfn "Event data: %s" data
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// For example, I can now create one click event, one key event, and
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// log both:
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let clickEvent = createClickEvent "Click on button"
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let keyEvent = createKeyEvent "Press 'Enter' key"
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logEvent clickEvent
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logEvent keyEvent
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// However, trying to mix or compare events of different types will
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// result in a compile-time error:
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// This line will not compile because clickEvent and keyEvent have
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// different phantom types.
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let result = clickEvent = keyEvent