Imperative computation in F# (I.) - Returning results from a function

One of the limitations of F# is that it doesn't very well support some of the advanced imperative language constructs such as break, continue or imperative style of returning value from a function, meaning that you can't write something like return false in the middle of the function. This has good reasons. F# doesn't in principle have the notion of currently executing statement and instead treat every code you write as an expression. Clearly, when there is no current statement, we cannot jump to other statements. If you're looking for more information about these basic principles, you can take a look at my book Real World Functional Programming, which covers this distinction in details in chapter 2, but we'll look at a brief example that will clarify this idea shortly.

Often, there is really no need to use break or other imperative constructs in F#, because you can write the same thing more elegantly using one of the provided higher order function such as Seq.exists or Seq.tryfind. However, there are still some cases where the imperative programming style makes it easier to express our original intention. Also, implementing your own higher order functions (akin to Seq.exists) would sometimes be much easier if we could just use imperative return.

So, what can be done about this? Adding imperative language features to F# seems a bit weird (at least to me), because it breaks the nice notion that everything is an expression, which makes it easy to reason about what the code does (again, there is an example of this in chapter 2 of my book). On the other hand, having these features would be really nice sometimes. In this article series, we'll look how to implement computation expression that allows us to write imperative code in F#. Computation expressions (an introduction is in chapter 12) allow us to create blocks of F# code that add some non-standard meaning to the usual F# code. In this first article, we'll allow returning from anywhere inside the body using return <expr>. In the next articles of the series, we'll look at more imperative constructs such as break and continue and we'll look even further and explore some features that aren't available in C#, but can be useful sometimes.

Programming with expressions

When programming in F#, everything is an expression. The easy way to understand what this means for a C# programmer is to imagine that we'd have to use conditional expressions (<expr>?<expr>:<expr>) instead of the usual if-then-else construct. Other constructs such as for loop would be also slightly different, but we don't need to go into the details. Now, C# allows you to use return as a statement anywhere, but you cannot use it as an expression, which means that the following isn't valid C# code:

return 
  (input == 1)
    ? (20 + 22) 
    : (4 * (return false));

The purpose of this example was just to demonstrate that disallowing imperative return in F# makes quite a good sense. The next point that I want to make before looking how to get imperative programming style in F# is that in many situations you don't really need to write the code in the imperative way. For example, let's say that you want to return true when some input collection of integers contains a number larger than 10 and false otherwise. The usual way to write this in C# would be something like this:

foreach(var n in input) 
  if (n > 10) return true;
return false;

There are various ways to write the same thing in F# - if you wanted to write this for the IEnumerable<T> type directly in F#, you couldn't use built-in for loop (because there is no way to return from it). This means that you'd have to call GetEnumerator explicitly and write a recursive function to do the processing. That sounds like a lot of pain. However, the F# library contains many generally useful higher order functions that make problems like this trivial. The previous example can be implemented in F# like this:

Seq.exists (fun n -> n > 10) input

As you can see, this is even easier than the original imperative solution. In fact, when using C# 3.0 and LINQ you would probably use one of the extension methods together with C# lambda expressions to write the code in a functional way even in C#. This means that before you'll think about using imperative style in F#, you should always check whether the same thing cannot be done in a lot simpler way using functions like Seq.fold, Seq.exists or Seq.tryfind. That said, we can now look how to simulate imperative programming in F# when we're really sure that we want it.

Imperative computations

When implementing F# computation expression, we need to create a builder object that contains a few primitive members that are used to compose the code from parts of the computations written by the user. We'll look at the implementation of the builder in a second. Once we'll have it - we'll call it ImperativeBuilder, we can write the following code to create imperative value that's used when writing all the imperative computations.

let imperative = new ImperativeBuilder()  

This one line of code created the builder, which we can now use to write imperative style of code in F#. We'll start by looking at some trivial example to demonstrate how the code works under the cover. Then we'll look at the implementation of the ImperativeBuilder and finally we'll write some more real world example. The following listing shows a single imperative function that contains the return statement two times. The behavior should be the same as C# - we want the first value to be returned and we don't want the printfn function to be ever called.

let test() = imperative {
  return 0
  printfn "after return!"
  return 1 }

When the F# compiler sees this code, it translates it into a couple of calls to primitives provided by the imperative builder. The most important primitives for us now are Return, which is used when translating the return keyword and Combine. The return keyword denotes the end of a sub-expression, so when we use it multiple times, the compiler needs some way to combine these sub-expressions into one and that's exactly when it uses the Combine primitive.

imperative.Run
 (imperative.Combine
   (imperative.Return(0),
    imperative.Delay(fun () ->
      printfn "after return"
      imperative.Return(1))))

There are a few other primitives that you can see in this example. We'll talk about them shortly when discussing the implementation. However, the important point that the original code that contained multiple return constructs (and felt like a sequence of statements) has been translated into a standard F# expression.

Designing the computation type

The key question when implementing computations like this one is what the type of the expression we've seen above is? We'll represent the computation using a type called Imperative<'T> and most of the primitives above work with this type. In particular, the Return primitive takes a value of some type - in our example above int and returns a value of type Imperative<int>. The Combine primitive takes two values of type Imperative<int> as an argument and returns a combined value of the same type.

Now, what should this type look like, so that we can implement the desired imperative behavior using the computation builder primitives? First of all, it must be a lazy type, which means that the printfn function shouldn't be called when evaluating the arguments for the Combine member. The easiest way to achieve this is to write the type as a function that takes a unit as an argument and returns something. This means that we don't have to run the second part of the code in the Combine primitive, when the first part already returned some value.

The second question is - what should be the value returned by this function? We must be able to represent computations that don't return any value (for example, when we have if-then loop without the else clause and the condition evaluates to false) and we need to represent computations that return something using the return primitive. The usual way to represent a value that may be missing in F# is to use the option<'T> type, so we'll end up with a type that looks like this:

type Imperative<'T> = unit -> option<'T>

This type can easily represent the two parts of the computation we've seen above. The first one (containing only return) will be represented as a function that immediately returns a value Some(0) and the second one will be represented as a function that prints string using the printfn function and then returns a value Some(1). A more complicated computation such as the if-then conditional without the else clause would be represented as a function that evaluates the condition; if the condition is true it runs the computation in the then clause and returns its result and otherwise returns None.

Implementing the computation

Now that we have the type that can represent the computations let's have a look at the implementation of the computation builder. We'll start by implementing the core primitives that were used in the example above. In the last section of the article, we'll also add support for two looping constructs that can be used inside computation expressions (for and while loops).

type ImperativeBuilder() = 
  // Creatae computation that returns the given value  
  member x.Return(v) : Imperative<_> = 
    (fun () -> Some(v))
  // Create computation that doesn't return any value
  member x.Zero() = (fun () -> None)

  // Return a computation that will evaluate the provided function  
  // only when the computation is being evaluated
  member x.Delay(f:unit -> Imperative<_>) = 
    (fun () -> f()())
  
  // Combines two delayed computations (that may return 
  // value imperatively using 'return') into one  
  member x.Combine(a, b) = (fun () ->
    // run the first part of the computation
    match a() with 
    // if it returned, we can return the result immediately
    | Some(v) -> Some(v) 
    // otherwise, we need to run the second part
    | _ -> b() )
  
  // Execute the imperative computation 
  // expression given as an argument
  member x.Run(imp) = 
    // run the computation and return the result or 
    // fail when the computation didn't return anything
    match imp() with 
    | Some(v) -> v 
    | None -> failwith "nothing returned!"

The first two members give us a way to construct primitive computations. The computation created using Return is used when you write return and represents a computation that returns something when executed. The second one represents a computation that doesn't do anything and is used in places where the F# compiler needs to fill in some blank space (for example when we don't provide the else clause). The Delay is also interesting. As we've seen when looking at the translation, it is used to wrap the second part of the computation when using Combine to make sure that the code which can do some side-effects (such as printing to the console) will be executed only when the second part of the computation is actually needed.

The first three primitives are used for creating individual parts of the computation that are later composed together into a single computation that represents the whole imperative block of code. This composition is done using the Combine primitive, which will get two computations as an argument. It returns a function (delayed computation) that, when executed, tries to run the first computation and returns its result if it returns a value. If the first computation doesn't return, it simply runs the second one and returns its result.

Finally, the computation builder also contains the Run primitive, which is used to wrap the whole computation. Inside this primitive, we run the computation to convert the internal representation (Imperative<'T>) into the actual value that we want to return ('T). When the computation returns None, we throw an exception because this means that the computation completed without ending with any return statement (we could also use Unchecked.defaultof<'T> to return zero or null if we wanted to simulate the behavior of C/C++, but we don't want to go that far in this article!) .

Real world example

Now that we've implemented the computation builder, we can take a look at some more realistic example where you may want to use imperative coding style. Most of the really interesting use cases will need imperative loops, so we'll get back to them soon, but even without loops, we can already write quite interesting code. The following listing shows a function that validates whether a provided string is a well-formed name. It uses various heuristics such as that the name should be at least two words, both reasonably long and both starting with an uppercase letter (By the way, sorry if your name doesn't pass the test - it's just a demo!)

> let validateName(arg:string) = imperative {
    // Should be non-empty and should contain space
    if (arg = null) then return false
    let idx = arg.IndexOf(" ")
    if (idx = -1) then return false
    
    // Verify the name and the surname
    let name = arg.Substring(0, idx)
    let surname = arg.Substring(idx + 1, arg.Length - idx - 1)
    if (surname.Length < 1 || name.Length < 1) then return false
    if (Char.IsLower(surname.[0]) || Char.IsLower(name.[0])) then return false

    // Looks like we've got a valid name!
    return true }
val validateName : string -> bool

> validateName(null);;
val it : bool = false
> validateName("Tomas");;
val it : bool = false
> validateName("Tomas Petricek");;
val it : bool = true

The code uses quite common imperative programming pattern. It implements various checks that are applied to the name and when the check fails, it immediately returns false as the result, otherwise it continues to the next check. In functional languages, this can be implemented either using nested if-then-else conditions or using pattern matching. If we wanted to avoid nesting in the second case, we'd have to move the entire check into the when clause, which would complicate the code. This means, that the code above is quite elegant solution for the problem and is in many ways cleaner than code you'd write using standard functional constructs.

In this example the difference between imperative and functional version would be mostly syntactical. I believe that syntax matters, so this is a good argument for me, but it may not be convincing example for everyone. In the next section, we'll add support for using imperative return inside loops, which will simplify solving some problems in a more significant way.

Supporting imperative loops

If we want to allow the user to write for and while loops inside computation expressions, we need to add two additional members to our computation builder. In most of the cases, these members can be implemented in terms of other primitives (such as Combine and Zero) by using the code you can see below. Note that exactly the same code would work for many other computation expressions, because we don't use the Imperative<'T> value directly. However, in the next articles of this series, we'll need to change this implementation a bit, so it is good that the F# language allows us to implement this ourselves.

type ImperativeBuilder with
  member x.For(inp:seq<_>, f) =
    // Process next element from the sequence
    let rec loop(en:IEnumerator<_>) = 
      // If ther are no more elements, return empty computation
      if not(en.MoveNext()) then x.Zero() else
        // Otherwise call body and combine it with a 
        // computation that continues looping
        x.Combine(f(en.Current), x.Delay(fun () -> loop(en)))
    // Start enumerating from the first element
    loop(inp.GetEnumerator())
    
  member x.While(gd, body) = 
    // Perform one step of the 'looping'
    let rec loop() =
      // If the condition is false, return empty computation
      if not(gd()) then x.Zero() else
        // Otherwise, call body and then loop again
        x.Combine(body, x.Delay(fun () -> loop()))
    loop()

We use type augmentation to add the two members to the existing ImperativeBuilder type. The implementation of both of the new primitives is quite similar. In both of the cases, we create a recursive function that performs one step of the looping and returns computation that should be executed after the step is performed. This means that the loop is 'unfolded' into a series of computation steps that are combined using the Combine primitive that we declared above. Note that we're also using the Delay primitive to make sure that the next step of the looping is run only when it is actually needed.

As discussed earlier, the Combine member may not need to run the second argument when the computation given as the first argument already returns some value (using return). For our looping primitives, this means that whenever one step of the looping runs the Return primitive, we'll stop looping and immediately return the result.

Returning from while loop

Now, let's have a look at how can we use the looping constructs to solve some real world problems. We'll start with the while loop and we'll use it to create a loop that reads names from the console and returns the first one that's valid:

> let readFirstName() = imperative {
    // Loop until the user enters valid name
    while true do
      let name = Console.ReadLine()
      // If the name is valid, we return it, otherwise
      // we continue looping...
      if (validateName(name)) then return name
      printfn "That's not a valid name! Try again..." }
val it : (unit -> string) = <fun:clo@0>

> readFirstName();;
Tomas
That's not a valid name! Try again...
Tomas Petricek
val it : string = "Tomas Petricek"

The listing starts by implementing the readFirstName function. If we look at its signature printed by the F# interactive, we can see that it returns string, which is exactly what we wanted. The function contains an infinite loop written using while true do loop, but because we can imperatively return from loops using return, it isn't actually an infinite loop. The function starts by reading a name from the console and then uses the function we created earlier to test whether the name is a valid name. If it is valid, then we want to return it as the result and if it is not, then we continue looping.

The interesting thing is, how the F# compiler translates the if expression. In the true case, it uses calls something like imperative.Return(name) to create an imperative computation that returns the name. We didn't provide any code to use for the false case, so the compiler uses the "empty computation" we provided and generates something like imperative.Zero(). As we've already seen, the Zero member returns a computation that doesn't yet have the return value. This means that when we finish executing the body of the loop and get back to the Combine primitive, it will execute the next iteration of the loop. On the other hand, if we return value using Return, the Combine primitive will see that we already have the return value, so it will stop looping.

Returning from for loop

To finish our discussion, let's look at one more real-world example where the imperative return is useful. In the beginning of the article, I wrote that we don't need to use imperative constructs in many of the cases, because the F# library contains higher order functions that make it possible to express the same thing (often even more concisely). However, implementing these higher order functions (such as Seq.exists) isn't as easy as it could be. For example, the exists function that takes a sequence as an argument has to be implemented by directly using GetEnumerator.

The following example shows that implementing exists function taking a sequence and a predicate as arguments using our imperative computation is very easy, because we can simply use for loop to enumerate over the source sequence. When the predicate returns true for the first time, we can immediately terminate the execution and return true as the overall result.

> let exists f inp = imperative {
    for v in inp do 
      if f(v) then return true
    return false }
val exists : ('a -> bool) -> seq<'a> -> bool

> [ 1 .. 10 ] |> exists (fun v -> v % 3 = 0)
val it : bool = true

As you can see, the implementation of the function couldn't be easier. If the predicate returns true for any of the elements, we imperatively return true and if all the elements are enumerated without finding any element like that, we come to the last line of the function, which returns false. It is important to note that in the example above, the predicate is executed only for elements 1, 2 and 3, because when we reach 3, we can return true without accessing the rest of the list.

Summary

In this article, we implemented the first version of the imperative computation builder that allows us to use imperative return statement that returns the result from any place in the function. We started with a relatively simple version and then we extended it to also support two standard F# looping constructs - for and while. We also looked at various examples that demonstrate situations where the imperative programming style allows us to write some code in easier way than when implementing the same functionality in the purely functional way.

However, there are more imperative constructs that you may sometimes need than just imperative return. In particular, many of the F# users sometimes ask for break and continue. Indeed, both of these can be added to our computation builder. We'll need to do a minor change to the way we represent the computation, but as you'll see in the next article of this series, it is definitely possible to do that and it can be done in a very syntactically convenient way.

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