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Option.fold() Considered Unreadable

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Option.fold() Considered Unreadable

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We had a lengthy discussion recently during code review whether  scala.Option.fold() is idiomatic and clever or maybe unreadable and tricky? Let's first describe what the problem is.  Option.fold does two things: maps a function  f over Option's value (if any) or returns an alternative  alt if it's absent. Using simple pattern matching we can implement it as follows:

val option: Option[T] = //...
def alt: R = //...
def f(in: T): R = //...

val x: R = option match {
    case Some(v) => f(v)
    case None => alt
}
If you prefer one-liner,  fold is actually a combination of  map and  getOrElse

val x: R = option map f getOrElse alt
Or, if you are a C programmer that still wants to write in C, but using Scala compiler:

val x: R = if (option.isDefined)
    f(option.get)
else
    alt
Interestingly this is similar to how  fold() is actually implemented, but that's an implementation detail. OK, all of the above can be replaced with single  Option.fold():

val x: R = option.fold(alt)(f)
Technically you can even use  /: and  \: operators ( alt /: option) - but that would be simply masochistic. I have three problems with  option.fold() idiom. First of all - it's anything but readable. We are folding (reducing) over  Option - which doesn't really make much sense. Secondly it reverses the ordinary  positive-then-negative-case flow by starting with failure (absence,  alt) condition followed by presence block ( f function; see also:  Refactoring map-getOrElse to fold). Interestingly this method would work great for me if it was named  mapOrElse:

**
 * Hypothetical in Option
 */
def mapOrElse[B](f: A => B, alt: => B): B =
    this map f getOrElse alt
Actually there is already such method in Scalaz, called  OptionW.catacata. Here is what Martin Odersky has to say about it:

"I personally find methods like  cata that take two closures as arguments are often overdoing it. Do you really gain in readability over  map +  getOrElse? Think of a newcomer to your code[...]"While  cata has some  theoretical backgroundOption.fold just sounds like a random name collision that doesn't bring anything to the table, apart from confusion. I know what you'll say, that  TraversableOnce has  fold and we are sort-of doing the same thing. Why it's a random collision rather than extending the contract described in TraversableOncefold() method in Scala collections typically just delegates to one of foldLeft()/ foldRight() (the one that works better for given data structure), thus it doesn't guarantee order and folding function has to be associative. But in Option.fold() the contract is different: folding function takes just one parameter rather than two. If you read  my previous article about folds you know that reducing function always takes two parameters: current element and accumulated value (initial value during first iteration). But  Option.fold() takes just one parameter: current  Option value! This breaks the consistency, especially when realizing  Option.foldLeft() and Option.foldRight() have  correct contract (but it doesn't mean they are more readable).

The only way to understand folding over option is to imagine  Option as a sequence with 0 or  1 elements. Then it sort of makes sense, right? No.

def double(x: Int) = x * 2

Some(21).fold(-1)(double)   //OK: 42
None.fold(-1)(double)       //OK: -1
but:

Some(21).toList.fold(-1)(double)
<console>: error: type mismatch;
 found   : Int => Int
 required: (Int, Int) => Int
              Some(21).toList.fold(-1)(double)
                                       ^
If we treat  Option[T] as a  List[T], awkward  Option.fold() breaks because it has different type than  TraversableOnce.fold(). This is my biggest concern. I can't understand why  folding wasn't defined in terms of the type system (trait?) and implemented strictly. As an example take a look at:

Data.Foldable in Haskell (advanced)

Data.Foldable typeclass describes various flavours of folding in Haskell. There are familiar  foldl/ foldr/ foldl1/ foldr1, in Scala named foldLeft/ foldRight/ reduceLeft/ reduceRight accordingly. They have the same type as Scala and behave unsurprisingly with all types that you can fold over, including  Maybe, lists, arrays, etc. There is also a function named  fold, but it has a completely different meaning:

class Foldable t where
    fold :: Monoid m => t m -> m
While other folds are quite complex, this one barely takes a foldable container of  ms (which have to be  Monoids) and returns the same  Monoid type. A quick recap: a type can be a Monoid if there exists a neutral value of that type and an operation that takes two values and produces just one. Applying that function with one of the arguments being neutral value yields the other argument.  String ( [Char]) is a good example with empty string being neutral value ( mempty) and string concatenation being such operation ( mappend). Notice that there are two different ways you can construct monoids for numbers: under addition with neutral value being  0 ( x + 0 == 0 + x == x for any  x) and under multiplication with neutral  1 ( x * 1 == 1 * x == x for any  x). Let's stick to strings. If I fold empty list of strings, I'll get an empty string. But when a list contains many elements, they are being concatenated:

> fold ([] :: [String])
""
> fold [] :: String
""
> fold ["foo", "bar"]
"foobar"
In the first example we have to explicitly say what is the type of empty list  []. Otherwise Haskell compiler can't figure out what is the type of elements in a list, thus which monoid instance to choose. In second example we declare that whatever is returned from  fold [], it should be a  String. From that the compiler infers that  [] actually must have a type of  [String]. Last  fold is the simplest: the program folds over elements in list and concatenates them because concatenation is the operation defined in  Monoid Stringtypeclass instance.

Back to options (or more precisely  Maybe). Folding over  Maybe monad having type parameter being  Monoid (I can't believe I just said it) has an interesting interpretation: it either returns value inside  Maybe or a default  Monoid value:

> fold (Just "abc")
"abc"
> fold Nothing :: String
""
Just "abc" is same as  Some("abc") in Scala. You can see here that if  Maybe Stringis  Nothing, neutral  String monoid value is returned, that is an empty string.

Summary

Haskell shows that folding (also over  Maybe) can be at least consistent. In Scala Option.fold is unrelated to  List.fold, confusing and unreadable. I advise avoiding it and staying with slightly more verbose  map/ getOrElse transformations or pattern matching.

PS: Did I mention there is also  Either.fold() (with even different contract) but no Try.fold()

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