I just got this lovely error message from the Haskell compiler:

./build-dbg Engine
[41 of 41] Compiling Engine           ( Engine.hs, dbg/Engine.o )

Engine.hs:147:5:
    My brain just exploded
    I can't handle pattern bindings for existential or GADT data constructors.
    Instead, use a case-expression, or do-notation, to unpack the constructor.
    In the pattern: BPPBox bProb
    In a pattern binding: BPPBox bProb = mkProb dir
    In the expression:
      let
        mkProb = inventory M.! problemName
        BPPBox bProb = mkProb dir
      in
        do { checkProblemName problemName;
             createDirectoryIfMissing True dir;
             msgInfo
               ("Flag to keep dead genomes initialized to " ++ show keepDead);
             .... }