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