This repository contains a Haskell library for Candid to Haskell. You can find the documentation for the library on https://nomeata.github.io/haskell-candid/Codec-Candid.html.
The focus here is correctness with regard to the spec, and maybe also convenience (e.g. it can guess the textual form of field names, which are just numbers on the wires), but not necessarily high performance. Let me know if you run into troubles because of this.
This package also comes with a hcandid command line tool, to convert between binary and textual candid. Just run hcandid --help to see how to use it.