Source: https://github.com/mcandre/genetics With {-# LANGUAGE TypeSynonymInstances #-} , my genetic algorithm code compiles and runs perfectly. $ make $ ./helloga ... hellowobld helloworyd helloporld hellyworld helloworrd hellowofld hellpworld Best candidate: helloworld Without it, Haskell refuses to declare an instance on Strings. $ make ghc --make -O2 -fforce-recomp helloga.hs [1 of 2] Compiling Genetics ( Genetics.hs, Genetics.o ) [2 of 2] Compiling Main ( helloga.hs, helloga.o ) helloga.hs:27:10: Illegal instance declaration for `Gene String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Gene String' make: *** [all] Error 1 The fix is easy to discover and apply, but this is my first typeclass, nothing complicated. As a newbie I prefer that it "just works" without my having to use the special compile flag. Cheers, Andrew Pennebaker www.yellosoft.us