My view of FP is that it's not compatible with the concept of modularity or separation of concerns. If your priority is to ensure that similar concepts are gro

My view of FP is that it's not compatible with the concept of modularity or separation of concerns. If your priority is to ensure that similar concepts are grouped together inside flexible and composable modules, then co-locating related state and logic is necessary. Unless a module can fully encapsulate (have full ownership over) the state which it operates on, it cannot fully separate its concerns from that of other modules. If you have a global state store and any module can potentially read from and write to any part of that state, then you are bound to end up with modules which have overlapping responsibilities - These 'modules' won't be very modular or composable because, whenever multiple modules share the same state, they invariably end up having to have an awareness of each other's behaviors (to avoid writing conflicting state changes or to know when they should update themselves). Underlying FP philosophy is the notion that there is no value in encapsulation; that there is no value in assigning exclusive ownership over specific state to a specific module; that such ownership is dangerous because it makes the module unpredictable from outside. What FP proponents fail to realize is that once they've rejected the philosophy of state encapsulation, they've implicitly accepted the philosophy of state multi-ownership. Whenever you have shared ownership of state, it becomes unclear who is responsible for that state; this leads to potential conflicts. Avoiding conflicts forces modules to become aware of each other's existence and interactions and this reduces their modularity. The functional programmer's need for modules to be fully predictable with each interaction was born out of a need to micromanage them. On the other hand, OOP proponents understand the value of module autonomy and see them as black-boxes which need to be trusted to do their work. OOP proponents understand the value of simple interfaces when it comes to modularity and they understand that such simple interfaces are only possible if the module has sufficient autonomy and this requires it to have full control over its state. An analogy for FP in the real world would be like a computer manufacturer relying on chips from a chip manufacturer, but the computer manufacturer insists that the chip manufacturer must produce chips using only the materials provided by the computer manufacturer... Let's say that, eventually, a new superior semiconductor material is discovered; the chip manufacturer will not be able to use it to improve their chips. Although it has autonomy over its manufacturing processes, it is constrained to the materials provided to it by the manufacturing company; therefore it cannot take advantage of the innovation behind the scenes without the computer manufacturer also updating its processes and materials. The chip manufacturer is predictable but it is not modular; it cannot improve itself without involvement from the computer manufacturer.