Category on NSArray to add functional operators

NSArray+Functional.h

#import <Foundation/Foundation.h>


@interface NSArray (Functional)

-(NSArray *) dropFirst;
-(NSArray *) dropLast;

-(NSArray *) map:(id (^)(id element))closure;

-(id) reduceWithInitial:(id)initial partial:(id (^)(id acc, id element))partial;

-(NSArray *) filter:(BOOL (^)(id element))shouldKeep;

@end

NSArray+Functional.m

#import "NSArray+Functional.h"

@implementation NSArray (Functional)

-(NSArray *) dropFirst {
    if(self.count == 0) {
        return @[];
    }
    
    return [self subarrayWithRange:NSMakeRange(1, self.count-1)];
}

-(NSArray *)dropLast {
    if(self.count == 0) {
        return @[];
    }
    
    return [self subarrayWithRange:NSMakeRange(0, self.count-1)];
}

-(NSArray *) map:(id (^)(id))closure {
    NSMutableArray *mutable = [NSMutableArray arrayWithCapacity:self.count];
    for (id element in self) {
        [mutable addObject:closure(element)];
    }
    return mutable;
}

-(id) reduceWithInitial:(id)initial partial:(id (^)(id acc, id element))partial {
    id acc = initial;
    for (id element in self) {
        acc = partial(acc, element);
    }
    return acc;
}

-(NSArray *) filter:(BOOL (^)(id element))shouldKeep {
    NSMutableArray *mutable = [NSMutableArray arrayWithCapacity:self.count];
    for (id element in self) {
        if(shouldKeep(element)) {
            [mutable addObject:element];
        }
    }
    return mutable;
}

@end

NSArrayFunctionalTests.m

#import <XCTest/XCTest.h>
#import "NSArray+Functional.h"

@interface NSArrayFunctionalTests : XCTestCase

@end

@implementation NSArrayFunctionalTests

// MARK: dropFirst

-(void) testDropFirstOnEmptyArray {
    XCTAssertEqualObjects([[NSArray array] dropFirst], @[]);
}

-(void) testDropFirstOnOneElementArray {
    NSArray *oneElementArray = @[@"one"];
    XCTAssertEqualObjects([oneElementArray dropFirst], @[]);
}

-(void) testDropFirst {
    NSArray *strings = @[@"one", @"two", @"three"];
    
    NSArray *expected = @[@"two", @"three"];
    XCTAssertEqualObjects([strings dropFirst], expected);
}

// MARK: dropLast

-(void) testDropLastOnEmptyArray {
    XCTAssertEqualObjects([[NSArray array] dropLast], @[]);
}

-(void) testDropLastOnOneElementArray {
    NSArray *oneElementArray = @[@"one"];
    XCTAssertEqualObjects([oneElementArray dropLast], @[]);
}

-(void) testDropLast {
    NSArray *strings = @[@"one", @"two", @"three"];
    
    NSArray *expected = @[@"one", @"two"];
    XCTAssertEqualObjects([strings dropLast], expected);
}

// MARK: map

-(void) testMapStrings {
    NSArray *strings = @[@"one", @"two", @"three"];

    NSArray *mappedStrings = [strings map:^id (NSString *obj) {
        return [obj uppercaseString];
    }];
    
    NSArray *expected = @[@"ONE", @"TWO", @"THREE"];
    XCTAssertEqualObjects(mappedStrings, expected);
}

// MARK: reduce

-(void) testReduceNumbers {
    NSArray *numbers = @[@(1), @(2), @(3)];
    
    NSArray *sum = [numbers reduceWithInitial:@(0) partial:^id(id acc, id element) {
        return [NSNumber numberWithInteger:([acc integerValue] + [element integerValue])];
    }];
    
    XCTAssertEqualObjects(sum, @(6));
}

-(void) testReduceStrings {
    NSArray *strings = @[@"one", @"two", @"three"];
    
    NSArray *concatenation = [[strings dropFirst] reduceWithInitial:strings.firstObject partial:^id(NSString *acc, NSString *element) {
        return [acc stringByAppendingFormat:@" %@", element];
    }];
    
    XCTAssertEqualObjects(concatenation, @"one two three");
}

// MARK: filter

-(void) testFiltersNumbers {
    NSArray *numbers = @[@(1), @(2), @(3), @(4), @(5), @(6), @(7), @(8)];
    
    NSArray *evenNumbers = [numbers filter:^BOOL(NSNumber *number) {
        return [number integerValue] % 2 == 0;
    }];
    
    NSArray *expectedNumbers = @[@(2), @(4), @(6), @(8)];
    XCTAssertEqualObjects(evenNumbers, expectedNumbers);
}

-(void) testFiltersStrings {
    NSArray *nouns = @[@"apple", @"cars", @"toys", @"skin"];
    
    NSArray *plurals = [nouns filter:^BOOL(NSString *noun) {
        return [noun hasSuffix:@"s"];
    }];
    
    NSArray *expected = @[@"cars", @"toys"];
    XCTAssertEqualObjects(plurals, expected);
}

@end