[Haskell-cafe] Use cases of empty type classes

Tomas Tauber tomtau at connect.hku.hk
Tue Mar 8 06:10:33 UTC 2016

Hi everyone,

I have one question. What are current use cases of type classes with no methods?

I saw early uses in type-level programming (e.g. HList [1]).
In the OO world, interfaces with no methods are called marker interfaces -- their use cases range from things that could be done with datatype generic programming in Haskell (e.g. serialization) to metadata annotations (e.g. RandomAccess [2]).


Tomas Tauber

[1] http://okmij.org/ftp/Haskell/HList-ext.pdf
[2] https://docs.oracle.com/javase/8/docs/api/java/util/RandomAccess.html

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