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Phantom type

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Revision as of 06:16, 6 February 2006

A phantom type is a type used only to construct other types; its values are never used. Phantom types are used in type arithmetic, for example.

An extension to Haskell 98 supported by GHC allows you to define datatypes without any constructors (and therefore no values other than bottom):

data MyType

This lets the compiler recognize phantom types and ensure they aren't used improperly.