> Eta reduction/expansion has nothing to do with performance. It affects inlining. GHC only inlines fully applied functions, where "fully applied" means applie

> Eta reduction/expansion has nothing to do with performance. It affects inlining. GHC only inlines fully applied functions, where "fully applied" means applied to as many arguments as it was declared with. So in something like `g = fmap f` your first definition of `f` would never get inlined into g whereas your second definition might.