Aelve Guide | Haskell

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In recent versions of GHC there is a notion of roles (see GHC Guide).

Types that have the same representation (e.g. a type and its newtype) can be converted between each other with coerce or with -XGeneralizedNewtypeDeriving. Roles exist to constrain ways in which types can be converted. For instance, this definition will disallow converting Set a to Set b even if b is a newtype of a:

type role Set nominal

The reason it's not safe to convert a Set is that the set will have a different structure depending on the Ord instance of what's stored inside, and instances can be different for newtypes even if the representations are the same.

There are three roles:

• nominal: types must be completely identical after type family reduction
• representational: types must have the same representation
• phantom: the type can be disregarded

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See GHC Guide on deriving strategies.

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