Scala flatMap and for comprehension

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Introduction I’m not a scala mega mind so feel free to correct me, but this is how I explain the flatMap/map/for-comprehension saga to myself!

To understand for comprehension and it’s translation to scala's map / flatMap we must take small steps and understand the composing parts - map and flatMap. But isn’t scala's flatMap just map with flatten you ask thyself! if so why do so many developers find it so hard to get the grasp of it or of for-comprehension / flatMap / map. Well, if you just look at scala’s map and flatMap signature you see they return the same return type M[B] and they work on the same input argument A (at least the first part to the function they take) if that’s so what makes a difference?

Our plan

  1. Understand scala’s map.
  2. Understand scala’s flatMap.
  3. Understand scala’s for comprehension.`

Scala’s map

scala map signature:

map[B](f: (A) => B): M[B]

But there is a big part missing when we look at this signature, and it’s - where does this A comes from? our container is of type A so its important to look at this function in the context of the container - M[A]. Our container could be a List of items of type A and our map function takes a function which transform each items of type A to type B, then it returns a container of type B (or M[B])

Let’s write map’s signature taking into account the container:

M[A]: // We are in M[A] context.
    map[B](f: (A) => B): M[B] // map takes a function which knows to transform A to B and then it bundles them in M[B]

Note an extremely highly highly important fact about map - it bundles automatically in the output container M[B] you have no control over it. Let’s us stress it again:

  1. map chooses the output container for us and its going to be the same container as the source we work on so for M[A] container we get the same M container only for B M[B] and nothing else!
  2. map does this containerization for us we just give a mapping from A to B and it would put it in the box of M[B] will put it in the box for us!

You see you did not specify how to containerize the item you just specified how to transform the internal items. And as we have the same container M for both M[A] and M[B] this means M[B] is the same container, meaning if you have List[A] then you are going to have a List[B] and more importantly map is doing it for you!

Now that we have dealt with map let’s move on to flatMap.

Scala’s flatMap

Let’s see its signature:

flatMap[B](f: (A) => M[B]): M[B] // we need to show it how to containerize the A into M[B]

You see the big difference from map to flatMap in flatMap we are providing it with the function that does not just convert from A to B but also containerizes it into M[B].

why do we care who does the containerization?

So why do we so much care of the input function to map/flatMap does the containerization into M[B] or the map itself does the containerization for us?

You see in the context of for comprehension what’s happening is multiple transformations on the item provided in the for so we are giving the next worker in our assembly line the ability to determine the packaging. imagine we have an assembly line each worker does something to the product and only the last worker is packaging it in a container! welcome to flatMap this is it’s purpose, in map each worker when finished working on the item also packages it so you get containers over containers.

The mighty for comprehension

Now let’s looks into your for comprehension taking into account what we said above:

def bothMatch(pat:String,pat2:String,s:String):Option[Boolean] = for {
    f <- mkMatcher(pat)   
    g <- mkMatcher(pat2)
} yield f(s) && g(s)

What have we got here:

  1. mkMatcher returns a container the container contains a function: String => Boolean
  2. The rules are the if we have multiple <- they translate to flatMap except for the last one.
  3. As f <- mkMatcher(pat) is first in sequence (think assembly line) all we want out of it is to take f and pass it to the next worker in the assembly line, we let the next worker in our assembly line (the next function) the ability to determine what would be the packaging back of our item this is why the last function is map.
  4. The last g <- mkMatcher(pat2) will use map this is because its last in assembly line! so it can just do the final operation with map( g => which yes! pulls out g and uses the f which has already been pulled out from the container by the flatMap therefore we end up with first:

    mkMatcher(pat) flatMap (f // pull out f function give item to next assembly line worker (you see it has access to f, and do not package it back i mean let the map determine the packaging let the next assembly line worker determine the container. mkMatcher(pat2) map (g => f(s) …)) // as this is the last function in the assembly line we are going to use map and pull g out of the container and to the packaging back, its map and this packaging will throttle all the way up and be our package or our container, yah!

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