Mutating index state variables, and it's not functional (you can't pass a function to be the inner loop)

For loops require explicit and error-prone use of integer indices and are of limited utility outside arrays. Foreach loops in Java/C# are an attempt to generalise, but they are really just syntactic sugar over external iterators. They give privileged status to the specific iteration interface chosen, entangling the language and its libraries. The actual Iterator interface chosen by Java has proven to be unsuitable for coroutines/yield, but is now too difficult to change. In Haskell, foreach loops can be implemented purely as a higher-order library function.