FunctionalBankAccount: a simple implementation of a blocks-based object model in (Objective-)C, inspired by a similar Scheme implementation in "Structure and Interpretation of Computer Programs". See http://oleb.net/blog/2013/02/building-blocks-based-object-system-in-objective-c/ for an explanation.

FunctionalBankAccount.m

#import <Foundation/Foundation.h>

typedef id(^BankAccount)(char *cmd);
typedef id(^CurrentBalanceMethod)(void);
typedef id(^DepositMethod)(double);
typedef id(^WithdrawMethod)(double);

BankAccount CreateBankAccount(double initialBalance)
{
    // Initialization
    NSCAssert(initialBalance >= 0.0, @"initialBalance must not be negative");
    double __block balance = initialBalance;

    // "Method Definitions"
    CurrentBalanceMethod currentBalance = ^id{
        return @(balance);
    };
    
    DepositMethod deposit = ^id(double depositAmount)
    {
        NSCAssert(depositAmount > 0.0, @"depositAmount must be greater than zero");
        balance = balance + depositAmount;
        return @(balance);
    };
    
    WithdrawMethod withdraw = ^id(double withdrawAmount)
    {
        NSCAssert(withdrawAmount > 0.0, @"withdrawAmount must be greater than zero");
        BOOL hasSufficientBalance = (balance >= withdrawAmount);
        if (hasSufficientBalance) {
            balance = balance - withdrawAmount;
            return @(balance);
        } else {
            return [NSError errorWithDomain:@"BankAccountErrorDomain" code:2 userInfo:@{ NSLocalizedDescriptionKey : @"Insufficient balance" }];
        }
    };

    // Message Dispatch
    id bankAccountBlock = ^id(char *cmd)
    {
        if (strcmp(cmd, "currentBalance") == 0) {
            return currentBalance;
        } else if (strcmp(cmd, "deposit") == 0) {
            return deposit;
        } else if (strcmp(cmd, "withdraw") == 0) {
            return withdraw;
        } else {
            return [NSError errorWithDomain:@"BankAccountErrorDomain" code:1 userInfo:@{ NSLocalizedDescriptionKey : @"Unknown command" }];
        }
    };
    
    return bankAccountBlock;
}


int main(int argc, const char * argv[])
{
    @autoreleasepool {
        BankAccount account = CreateBankAccount(100);
        NSNumber *balance = ((CurrentBalanceMethod)account("currentBalance"))();
        NSLog(@"Balance: %@", balance);
        balance = ((DepositMethod)account("deposit"))(50);
        NSLog(@"Depositing 50, new balance: %@", balance);
        balance = ((WithdrawMethod)account("withdraw"))(30);
        NSLog(@"Withdrawing 30, new balance: %@", balance);
        balance = ((WithdrawMethod)account("withdraw"))(100);
        NSLog(@"Withdrawing 100, new balance: %@", balance);
    }
    return 0;
}

Check out my accompanying blog post.