You're probably thinking of the original version of Cap'n Proto's schema parser, which was written in Haskell. The code generation step that it helped to handle is not a part of this benchmark.
As far as I know, nobody has written a Cap'n Proto serialization runtime for Haskell yet.
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I wish we could compare the old Haskell version to the C++ and Rust versions as well.
You're probably thinking of the original version of Cap'n Proto's schema parser, which was written in Haskell. The code generation step that it helped to handle is not a part of this benchmark.
As far as I know, nobody has written a Cap'n Proto serialization runtime for Haskell yet.
See http://kentonv.github.io/capnproto/otherlang.html .